fs: introduce write_begin, write_end, and perform_write aops

These are intended to replace prepare_write and commit_write with more
flexible alternatives that are also able to avoid the buffered write
deadlock problems efficiently (which prepare_write is unable to do).

[mark.fasheh@oracle.com: API design contributions, code review and fixes]
[akpm@linux-foundation.org: various fixes]
[dmonakhov@sw.ru: new aop block_write_begin fix]
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Signed-off-by: Dmitriy Monakhov <dmonakhov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/Locking

@@ -178,15 +178,18 @@ prototypes:
 locking rules:
 	All except set_page_dirty may block
 
-			BKL	PageLocked(page)
+			BKL	PageLocked(page)	i_sem
 writepage:		no	yes, unlocks (see below)
 readpage:		no	yes, unlocks
 sync_page:		no	maybe
 writepages:		no
 set_page_dirty		no	no
 readpages:		no
-prepare_write:		no	yes
-commit_write:		no	yes
+prepare_write:		no	yes			yes
+commit_write:		no	yes			yes
+write_begin:		no	locks the page		yes
+write_end:		no	yes, unlocks		yes
+perform_write:		no	n/a			yes
 bmap:			yes
 invalidatepage:		no	yes
 releasepage:		no	yes

Documentation/filesystems/vfs.txt

@@ -537,6 +537,12 @@ struct address_space_operations {
 			struct list_head *pages, unsigned nr_pages);
 	int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
 	int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
+	int (*write_begin)(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned flags,
+				struct page **pagep, void **fsdata);
+	int (*write_end)(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned copied,
+				struct page *page, void *fsdata);
 	sector_t (*bmap)(struct address_space *, sector_t);
 	int (*invalidatepage) (struct page *, unsigned long);
 	int (*releasepage) (struct page *, int);
@@ -633,6 +639,45 @@ struct address_space_operations {
         operations.  It should avoid returning an error if possible -
         errors should have been handled by prepare_write.
 
+  write_begin: This is intended as a replacement for prepare_write. The
+	key differences being that:
+		- it returns a locked page (in *pagep) rather than being
+		  given a pre locked page;
+		- it must be able to cope with short writes (where the
+		  length passed to write_begin is greater than the number
+		  of bytes copied into the page).
+
+	Called by the generic buffered write code to ask the filesystem to
+	prepare to write len bytes at the given offset in the file. The
+	address_space should check that the write will be able to complete,
+	by allocating space if necessary and doing any other internal
+	housekeeping.  If the write will update parts of any basic-blocks on
+	storage, then those blocks should be pre-read (if they haven't been
+	read already) so that the updated blocks can be written out properly.
+
+        The filesystem must return the locked pagecache page for the specified
+	offset, in *pagep, for the caller to write into.
+
+	flags is a field for AOP_FLAG_xxx flags, described in
+	include/linux/fs.h.
+
+        A void * may be returned in fsdata, which then gets passed into
+        write_end.
+
+        Returns 0 on success; < 0 on failure (which is the error code), in
+	which case write_end is not called.
+
+  write_end: After a successful write_begin, and data copy, write_end must
+        be called. len is the original len passed to write_begin, and copied
+        is the amount that was able to be copied (copied == len is always true
+	if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
+
+        The filesystem must take care of unlocking the page and releasing it
+        refcount, and updating i_size.
+
+        Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
+        that were able to be copied into pagecache.
+
   bmap: called by the VFS to map a logical block offset within object to
   	physical block number. This method is used by the FIBMAP
   	ioctl and for working with swap-files.  To be able to swap to

drivers/block/loop.c

@@ -204,14 +204,13 @@ lo_do_transfer(struct loop_device *lo, int cmd,
  * do_lo_send_aops - helper for writing data to a loop device
  *
  * This is the fast version for backing filesystems which implement the address
- * space operations prepare_write and commit_write.
+ * space operations write_begin and write_end.
  */
 static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
-		int bsize, loff_t pos, struct page *page)
+		int bsize, loff_t pos, struct page *unused)
 {
 	struct file *file = lo->lo_backing_file; /* kudos to NFsckingS */
 	struct address_space *mapping = file->f_mapping;
-	const struct address_space_operations *aops = mapping->a_ops;
 	pgoff_t index;
 	unsigned offset, bv_offs;
 	int len, ret;
@@ -223,63 +222,47 @@ static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
 	len = bvec->bv_len;
 	while (len > 0) {
 		sector_t IV;
-		unsigned size;
+		unsigned size, copied;
 		int transfer_result;
+		struct page *page;
+		void *fsdata;
 
 		IV = ((sector_t)index << (PAGE_CACHE_SHIFT - 9))+(offset >> 9);
 		size = PAGE_CACHE_SIZE - offset;
 		if (size > len)
 			size = len;
-		page = grab_cache_page(mapping, index);
-		if (unlikely(!page))
+
+		ret = pagecache_write_begin(file, mapping, pos, size, 0,
+							&page, &fsdata);
+		if (ret)
 			goto fail;
-		ret = aops->prepare_write(file, page, offset,
-					  offset + size);
-		if (unlikely(ret)) {
-			if (ret == AOP_TRUNCATED_PAGE) {
-				page_cache_release(page);
-				continue;
-			}
-			goto unlock;
-		}
+
 		transfer_result = lo_do_transfer(lo, WRITE, page, offset,
 				bvec->bv_page, bv_offs, size, IV);
-		if (unlikely(transfer_result)) {
-			/*
-			 * The transfer failed, but we still write the data to
-			 * keep prepare/commit calls balanced.
-			 */
-			printk(KERN_ERR "loop: transfer error block %llu\n",
-			       (unsigned long long)index);
-			zero_user_page(page, offset, size, KM_USER0);
-		}
-		flush_dcache_page(page);
-		ret = aops->commit_write(file, page, offset,
-					 offset + size);
-		if (unlikely(ret)) {
-			if (ret == AOP_TRUNCATED_PAGE) {
-				page_cache_release(page);
-				continue;
-			}
-			goto unlock;
-		}
+		copied = size;
 		if (unlikely(transfer_result))
-			goto unlock;
-		bv_offs += size;
-		len -= size;
+			copied = 0;
+
+		ret = pagecache_write_end(file, mapping, pos, size, copied,
+							page, fsdata);
+		if (ret < 0)
+			goto fail;
+		if (ret < copied)
+			copied = ret;
+
+		if (unlikely(transfer_result))
+			goto fail;
+
+		bv_offs += copied;
+		len -= copied;
 		offset = 0;
 		index++;
-		pos += size;
-		unlock_page(page);
-		page_cache_release(page);
+		pos += copied;
 	}
 	ret = 0;
 out:
 	mutex_unlock(&mapping->host->i_mutex);
 	return ret;
-unlock:
-	unlock_page(page);
-	page_cache_release(page);
 fail:
 	ret = -1;
 	goto out;
@@ -313,7 +296,7 @@ static int __do_lo_send_write(struct file *file,
  * do_lo_send_direct_write - helper for writing data to a loop device
  *
  * This is the fast, non-transforming version for backing filesystems which do
- * not implement the address space operations prepare_write and commit_write.
+ * not implement the address space operations write_begin and write_end.
  * It uses the write file operation which should be present on all writeable
  * filesystems.
  */
@@ -332,7 +315,7 @@ static int do_lo_send_direct_write(struct loop_device *lo,
  * do_lo_send_write - helper for writing data to a loop device
  *
  * This is the slow, transforming version for filesystems which do not
- * implement the address space operations prepare_write and commit_write.  It
+ * implement the address space operations write_begin and write_end.  It
  * uses the write file operation which should be present on all writeable
  * filesystems.
  *
@@ -780,7 +763,7 @@ static int loop_set_fd(struct loop_device *lo, struct file *lo_file,
 		 */
 		if (!file->f_op->splice_read)
 			goto out_putf;
-		if (aops->prepare_write && aops->commit_write)
+		if (aops->prepare_write || aops->write_begin)
 			lo_flags |= LO_FLAGS_USE_AOPS;
 		if (!(lo_flags & LO_FLAGS_USE_AOPS) && !file->f_op->write)
 			lo_flags |= LO_FLAGS_READ_ONLY;

fs/buffer.c

@@ -1770,6 +1770,48 @@ recover:
 	goto done;
 }
 
+/*
+ * If a page has any new buffers, zero them out here, and mark them uptodate
+ * and dirty so they'll be written out (in order to prevent uninitialised
+ * block data from leaking). And clear the new bit.
+ */
+void page_zero_new_buffers(struct page *page, unsigned from, unsigned to)
+{
+	unsigned int block_start, block_end;
+	struct buffer_head *head, *bh;
+
+	BUG_ON(!PageLocked(page));
+	if (!page_has_buffers(page))
+		return;
+
+	bh = head = page_buffers(page);
+	block_start = 0;
+	do {
+		block_end = block_start + bh->b_size;
+
+		if (buffer_new(bh)) {
+			if (block_end > from && block_start < to) {
+				if (!PageUptodate(page)) {
+					unsigned start, size;
+
+					start = max(from, block_start);
+					size = min(to, block_end) - start;
+
+					zero_user_page(page, start, size, KM_USER0);
+					set_buffer_uptodate(bh);
+				}
+
+				clear_buffer_new(bh);
+				mark_buffer_dirty(bh);
+			}
+		}
+
+		block_start = block_end;
+		bh = bh->b_this_page;
+	} while (bh != head);
+}
+EXPORT_SYMBOL(page_zero_new_buffers);
+
 static int __block_prepare_write(struct inode *inode, struct page *page,
 		unsigned from, unsigned to, get_block_t *get_block)
 {
@@ -1854,38 +1896,8 @@ static int __block_prepare_write(struct inode *inode, struct page *page,
 		if (!buffer_uptodate(*wait_bh))
 			err = -EIO;
 	}
-	if (!err) {
-		bh = head;
-		do {
-			if (buffer_new(bh))
-				clear_buffer_new(bh);
-		} while ((bh = bh->b_this_page) != head);
-		return 0;
-	}
-	/* Error case: */
-	/*
-	 * Zero out any newly allocated blocks to avoid exposing stale
-	 * data.  If BH_New is set, we know that the block was newly
-	 * allocated in the above loop.
-	 */
-	bh = head;
-	block_start = 0;
-	do {
-		block_end = block_start+blocksize;
-		if (block_end <= from)
-			goto next_bh;
-		if (block_start >= to)
-			break;
-		if (buffer_new(bh)) {
-			clear_buffer_new(bh);
-			zero_user_page(page, block_start, bh->b_size, KM_USER0);
-			set_buffer_uptodate(bh);
-			mark_buffer_dirty(bh);
-		}
-next_bh:
-		block_start = block_end;
-		bh = bh->b_this_page;
-	} while (bh != head);
+	if (unlikely(err))
+		page_zero_new_buffers(page, from, to);
 	return err;
 }
 
@@ -1910,6 +1922,7 @@ static int __block_commit_write(struct inode *inode, struct page *page,
 			set_buffer_uptodate(bh);
 			mark_buffer_dirty(bh);
 		}
+		clear_buffer_new(bh);
 	}
 
 	/*
@@ -1923,6 +1936,130 @@ static int __block_commit_write(struct inode *inode, struct page *page,
 	return 0;
 }
 
+/*
+ * block_write_begin takes care of the basic task of block allocation and
+ * bringing partial write blocks uptodate first.
+ *
+ * If *pagep is not NULL, then block_write_begin uses the locked page
+ * at *pagep rather than allocating its own. In this case, the page will
+ * not be unlocked or deallocated on failure.
+ */
+int block_write_begin(struct file *file, struct address_space *mapping,
+			loff_t pos, unsigned len, unsigned flags,
+			struct page **pagep, void **fsdata,
+			get_block_t *get_block)
+{
+	struct inode *inode = mapping->host;
+	int status = 0;
+	struct page *page;
+	pgoff_t index;
+	unsigned start, end;
+	int ownpage = 0;
+
+	index = pos >> PAGE_CACHE_SHIFT;
+	start = pos & (PAGE_CACHE_SIZE - 1);
+	end = start + len;
+
+	page = *pagep;
+	if (page == NULL) {
+		ownpage = 1;
+		page = __grab_cache_page(mapping, index);
+		if (!page) {
+			status = -ENOMEM;
+			goto out;
+		}
+		*pagep = page;
+	} else
+		BUG_ON(!PageLocked(page));
+
+	status = __block_prepare_write(inode, page, start, end, get_block);
+	if (unlikely(status)) {
+		ClearPageUptodate(page);
+
+		if (ownpage) {
+			unlock_page(page);
+			page_cache_release(page);
+			*pagep = NULL;
+
+			/*
+			 * prepare_write() may have instantiated a few blocks
+			 * outside i_size.  Trim these off again. Don't need
+			 * i_size_read because we hold i_mutex.
+			 */
+			if (pos + len > inode->i_size)
+				vmtruncate(inode, inode->i_size);
+		}
+		goto out;
+	}
+
+out:
+	return status;
+}
+EXPORT_SYMBOL(block_write_begin);
+
+int block_write_end(struct file *file, struct address_space *mapping,
+			loff_t pos, unsigned len, unsigned copied,
+			struct page *page, void *fsdata)
+{
+	struct inode *inode = mapping->host;
+	unsigned start;
+
+	start = pos & (PAGE_CACHE_SIZE - 1);
+
+	if (unlikely(copied < len)) {
+		/*
+		 * The buffers that were written will now be uptodate, so we
+		 * don't have to worry about a readpage reading them and
+		 * overwriting a partial write. However if we have encountered
+		 * a short write and only partially written into a buffer, it
+		 * will not be marked uptodate, so a readpage might come in and
+		 * destroy our partial write.
+		 *
+		 * Do the simplest thing, and just treat any short write to a
+		 * non uptodate page as a zero-length write, and force the
+		 * caller to redo the whole thing.
+		 */
+		if (!PageUptodate(page))
+			copied = 0;
+
+		page_zero_new_buffers(page, start+copied, start+len);
+	}
+	flush_dcache_page(page);
+
+	/* This could be a short (even 0-length) commit */
+	__block_commit_write(inode, page, start, start+copied);
+
+	return copied;
+}
+EXPORT_SYMBOL(block_write_end);
+
+int generic_write_end(struct file *file, struct address_space *mapping,
+			loff_t pos, unsigned len, unsigned copied,
+			struct page *page, void *fsdata)
+{
+	struct inode *inode = mapping->host;
+
+	copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
+
+	/*
+	 * No need to use i_size_read() here, the i_size
+	 * cannot change under us because we hold i_mutex.
+	 *
+	 * But it's important to update i_size while still holding page lock:
+	 * page writeout could otherwise come in and zero beyond i_size.
+	 */
+	if (pos+copied > inode->i_size) {
+		i_size_write(inode, pos+copied);
+		mark_inode_dirty(inode);
+	}
+
+	unlock_page(page);
+	page_cache_release(page);
+
+	return copied;
+}
+EXPORT_SYMBOL(generic_write_end);
+
 /*
  * Generic "read page" function for block devices that have the normal
  * get_block functionality. This is most of the block device filesystems.

fs/libfs.c

@@ -351,6 +351,26 @@ int simple_prepare_write(struct file *file, struct page *page,
 	return 0;
 }
 
+int simple_write_begin(struct file *file, struct address_space *mapping,
+			loff_t pos, unsigned len, unsigned flags,
+			struct page **pagep, void **fsdata)
+{
+	struct page *page;
+	pgoff_t index;
+	unsigned from;
+
+	index = pos >> PAGE_CACHE_SHIFT;
+	from = pos & (PAGE_CACHE_SIZE - 1);
+
+	page = __grab_cache_page(mapping, index);
+	if (!page)
+		return -ENOMEM;
+
+	*pagep = page;
+
+	return simple_prepare_write(file, page, from, from+len);
+}
+
 int simple_commit_write(struct file *file, struct page *page,
 			unsigned from, unsigned to)
 {
@@ -369,6 +389,28 @@ int simple_commit_write(struct file *file, struct page *page,
 	return 0;
 }
 
+int simple_write_end(struct file *file, struct address_space *mapping,
+			loff_t pos, unsigned len, unsigned copied,
+			struct page *page, void *fsdata)
+{
+	unsigned from = pos & (PAGE_CACHE_SIZE - 1);
+
+	/* zero the stale part of the page if we did a short copy */
+	if (copied < len) {
+		void *kaddr = kmap_atomic(page, KM_USER0);
+		memset(kaddr + from + copied, 0, len - copied);
+		flush_dcache_page(page);
+		kunmap_atomic(kaddr, KM_USER0);
+	}
+
+	simple_commit_write(file, page, from, from+copied);
+
+	unlock_page(page);
+	page_cache_release(page);
+
+	return copied;
+}
+
 /*
  * the inodes created here are not hashed. If you use iunique to generate
  * unique inode values later for this filesystem, then you must take care
@@ -642,6 +684,8 @@ EXPORT_SYMBOL(dcache_dir_open);
 EXPORT_SYMBOL(dcache_readdir);
 EXPORT_SYMBOL(generic_read_dir);
 EXPORT_SYMBOL(get_sb_pseudo);
+EXPORT_SYMBOL(simple_write_begin);
+EXPORT_SYMBOL(simple_write_end);
 EXPORT_SYMBOL(simple_commit_write);
 EXPORT_SYMBOL(simple_dir_inode_operations);
 EXPORT_SYMBOL(simple_dir_operations);

fs/namei.c

@@ -2729,53 +2729,29 @@ int __page_symlink(struct inode *inode, const char *symname, int len,
 {
 	struct address_space *mapping = inode->i_mapping;
 	struct page *page;
+	void *fsdata;
 	int err;
 	char *kaddr;
 
 retry:
-	err = -ENOMEM;
-	page = find_or_create_page(mapping, 0, gfp_mask);
-	if (!page)
-		goto fail;
-	err = mapping->a_ops->prepare_write(NULL, page, 0, len-1);
-	if (err == AOP_TRUNCATED_PAGE) {
-		page_cache_release(page);
-		goto retry;
-	}
+	err = pagecache_write_begin(NULL, mapping, 0, len-1,
+				AOP_FLAG_UNINTERRUPTIBLE, &page, &fsdata);
 	if (err)
-		goto fail_map;
+		goto fail;
+
 	kaddr = kmap_atomic(page, KM_USER0);
 	memcpy(kaddr, symname, len-1);
 	kunmap_atomic(kaddr, KM_USER0);
-	err = mapping->a_ops->commit_write(NULL, page, 0, len-1);
-	if (err == AOP_TRUNCATED_PAGE) {
-		page_cache_release(page);
-		goto retry;
-	}
-	if (err)
-		goto fail_map;
-	/*
-	 * Notice that we are _not_ going to block here - end of page is
-	 * unmapped, so this will only try to map the rest of page, see
-	 * that it is unmapped (typically even will not look into inode -
-	 * ->i_size will be enough for everything) and zero it out.
-	 * OTOH it's obviously correct and should make the page up-to-date.
-	 */
-	if (!PageUptodate(page)) {
-		err = mapping->a_ops->readpage(NULL, page);
-		if (err != AOP_TRUNCATED_PAGE)
-			wait_on_page_locked(page);
-	} else {
-		unlock_page(page);
-	}
-	page_cache_release(page);
+
+	err = pagecache_write_end(NULL, mapping, 0, len-1, len-1,
+							page, fsdata);
 	if (err < 0)
 		goto fail;
+	if (err < len-1)
+		goto retry;
+
 	mark_inode_dirty(inode);
 	return 0;
-fail_map:
-	unlock_page(page);
-	page_cache_release(page);
 fail:
 	return err;
 }

fs/splice.c

@@ -563,7 +563,7 @@ static int pipe_to_file(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 	struct address_space *mapping = file->f_mapping;
 	unsigned int offset, this_len;
 	struct page *page;
-	pgoff_t index;
+	void *fsdata;
 	int ret;
 
 	/*
@@ -573,49 +573,16 @@ static int pipe_to_file(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 	if (unlikely(ret))
 		return ret;
 
-	index = sd->pos >> PAGE_CACHE_SHIFT;
 	offset = sd->pos & ~PAGE_CACHE_MASK;
 
 	this_len = sd->len;
 	if (this_len + offset > PAGE_CACHE_SIZE)
 		this_len = PAGE_CACHE_SIZE - offset;
 
-find_page:
-	page = find_lock_page(mapping, index);
-	if (!page) {
-		ret = -ENOMEM;
-		page = page_cache_alloc_cold(mapping);
-		if (unlikely(!page))
-			goto out_ret;
-
-		/*
-		 * This will also lock the page
-		 */
-		ret = add_to_page_cache_lru(page, mapping, index,
-					    GFP_KERNEL);
-		if (unlikely(ret))
-			goto out_release;
-	}
-
-	ret = mapping->a_ops->prepare_write(file, page, offset, offset+this_len);
-	if (unlikely(ret)) {
-		loff_t isize = i_size_read(mapping->host);
-
-		if (ret != AOP_TRUNCATED_PAGE)
-			unlock_page(page);
-		page_cache_release(page);
-		if (ret == AOP_TRUNCATED_PAGE)
-			goto find_page;
-
-		/*
-		 * prepare_write() may have instantiated a few blocks
-		 * outside i_size.  Trim these off again.
-		 */
-		if (sd->pos + this_len > isize)
-			vmtruncate(mapping->host, isize);
-
-		goto out_ret;
-	}
+	ret = pagecache_write_begin(file, mapping, sd->pos, this_len,
+				AOP_FLAG_UNINTERRUPTIBLE, &page, &fsdata);
+	if (unlikely(ret))
+		goto out;
 
 	if (buf->page != page) {
 		/*
@@ -629,31 +596,9 @@ find_page:
 		kunmap_atomic(dst, KM_USER1);
 		buf->ops->unmap(pipe, buf, src);
 	}
-
-	ret = mapping->a_ops->commit_write(file, page, offset, offset+this_len);
-	if (ret) {
-		if (ret == AOP_TRUNCATED_PAGE) {
-			page_cache_release(page);
-			goto find_page;
-		}
-		if (ret < 0)
-			goto out;
-		/*
-		 * Partial write has happened, so 'ret' already initialized by
-		 * number of bytes written, Where is nothing we have to do here.
-		 */
-	} else
-		ret = this_len;
-	/*
-	 * Return the number of bytes written and mark page as
-	 * accessed, we are now done!
-	 */
-	mark_page_accessed(page);
+	ret = pagecache_write_end(file, mapping, sd->pos, this_len, this_len,
+				page, fsdata);
 out:
-	unlock_page(page);
-out_release:
-	page_cache_release(page);
-out_ret:
 	return ret;
 }
 

include/linux/buffer_head.h

@@ -203,6 +203,16 @@ void block_invalidatepage(struct page *page, unsigned long offset);
 int block_write_full_page(struct page *page, get_block_t *get_block,
 				struct writeback_control *wbc);
 int block_read_full_page(struct page*, get_block_t*);
+int block_write_begin(struct file *, struct address_space *,
+				loff_t, unsigned, unsigned,
+				struct page **, void **, get_block_t*);
+int block_write_end(struct file *, struct address_space *,
+				loff_t, unsigned, unsigned,
+				struct page *, void *);
+int generic_write_end(struct file *, struct address_space *,
+				loff_t, unsigned, unsigned,
+				struct page *, void *);
+void page_zero_new_buffers(struct page *page, unsigned from, unsigned to);
 int block_prepare_write(struct page*, unsigned, unsigned, get_block_t*);
 int cont_prepare_write(struct page*, unsigned, unsigned, get_block_t*,
 				loff_t *);

include/linux/fs.h

@@ -394,6 +394,8 @@ enum positive_aop_returns {
 	AOP_TRUNCATED_PAGE	= 0x80001,
 };
 
+#define AOP_FLAG_UNINTERRUPTIBLE	0x0001 /* will not do a short write */
+
 /*
  * oh the beauties of C type declarations.
  */
@@ -413,7 +415,7 @@ size_t iov_iter_copy_from_user_atomic(struct page *page,
 size_t iov_iter_copy_from_user(struct page *page,
 		struct iov_iter *i, unsigned long offset, size_t bytes);
 void iov_iter_advance(struct iov_iter *i, size_t bytes);
-int iov_iter_fault_in_readable(struct iov_iter *i);
+int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes);
 size_t iov_iter_single_seg_count(struct iov_iter *i);
 
 static inline void iov_iter_init(struct iov_iter *i,
@@ -454,6 +456,14 @@ struct address_space_operations {
 	 */
 	int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
 	int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
+
+	int (*write_begin)(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned flags,
+				struct page **pagep, void **fsdata);
+	int (*write_end)(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned copied,
+				struct page *page, void *fsdata);
+
 	/* Unfortunately this kludge is needed for FIBMAP. Don't use it */
 	sector_t (*bmap)(struct address_space *, sector_t);
 	void (*invalidatepage) (struct page *, unsigned long);
@@ -468,6 +478,18 @@ struct address_space_operations {
 	int (*launder_page) (struct page *);
 };
 
+/*
+ * pagecache_write_begin/pagecache_write_end must be used by general code
+ * to write into the pagecache.
+ */
+int pagecache_write_begin(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned flags,
+				struct page **pagep, void **fsdata);
+
+int pagecache_write_end(struct file *, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned copied,
+				struct page *page, void *fsdata);
+
 struct backing_dev_info;
 struct address_space {
 	struct inode		*host;		/* owner: inode, block_device */
@@ -1866,6 +1888,12 @@ extern int simple_prepare_write(struct file *file, struct page *page,
 			unsigned offset, unsigned to);
 extern int simple_commit_write(struct file *file, struct page *page,
 				unsigned offset, unsigned to);
+extern int simple_write_begin(struct file *file, struct address_space *mapping,
+			loff_t pos, unsigned len, unsigned flags,
+			struct page **pagep, void **fsdata);
+extern int simple_write_end(struct file *file, struct address_space *mapping,
+			loff_t pos, unsigned len, unsigned copied,
+			struct page *page, void *fsdata);
 
 extern struct dentry *simple_lookup(struct inode *, struct dentry *, struct nameidata *);
 extern ssize_t generic_read_dir(struct file *, char __user *, size_t, loff_t *);

include/linux/pagemap.h

@@ -96,6 +96,8 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t start,
 unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
 			int tag, unsigned int nr_pages, struct page **pages);
 
+struct page *__grab_cache_page(struct address_space *mapping, pgoff_t index);
+
 /*
  * Returns locked page at given index in given cache, creating it if needed.
  */

mm/filemap.c

@@ -1742,11 +1742,20 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
 	i->count -= bytes;
 }
 
-int iov_iter_fault_in_readable(struct iov_iter *i)
+/*
+ * Fault in the first iovec of the given iov_iter, to a maximum length
+ * of bytes. Returns 0 on success, or non-zero if the memory could not be
+ * accessed (ie. because it is an invalid address).
+ *
+ * writev-intensive code may want this to prefault several iovecs -- that
+ * would be possible (callers must not rely on the fact that _only_ the
+ * first iovec will be faulted with the current implementation).
+ */
+int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes)
 {
-	size_t seglen = min(i->iov->iov_len - i->iov_offset, i->count);
 	char __user *buf = i->iov->iov_base + i->iov_offset;
-	return fault_in_pages_readable(buf, seglen);
+	bytes = min(bytes, i->iov->iov_len - i->iov_offset);
+	return fault_in_pages_readable(buf, bytes);
 }
 
 /*
@@ -1843,6 +1852,95 @@ inline int generic_write_checks(struct file *file, loff_t *pos, size_t *count, i
 }
 EXPORT_SYMBOL(generic_write_checks);
 
+int pagecache_write_begin(struct file *file, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned flags,
+				struct page **pagep, void **fsdata)
+{
+	const struct address_space_operations *aops = mapping->a_ops;
+
+	if (aops->write_begin) {
+		return aops->write_begin(file, mapping, pos, len, flags,
+							pagep, fsdata);
+	} else {
+		int ret;
+		pgoff_t index = pos >> PAGE_CACHE_SHIFT;
+		unsigned offset = pos & (PAGE_CACHE_SIZE - 1);
+		struct inode *inode = mapping->host;
+		struct page *page;
+again:
+		page = __grab_cache_page(mapping, index);
+		*pagep = page;
+		if (!page)
+			return -ENOMEM;
+
+		if (flags & AOP_FLAG_UNINTERRUPTIBLE && !PageUptodate(page)) {
+			/*
+			 * There is no way to resolve a short write situation
+			 * for a !Uptodate page (except by double copying in
+			 * the caller done by generic_perform_write_2copy).
+			 *
+			 * Instead, we have to bring it uptodate here.
+			 */
+			ret = aops->readpage(file, page);
+			page_cache_release(page);
+			if (ret) {
+				if (ret == AOP_TRUNCATED_PAGE)
+					goto again;
+				return ret;
+			}
+			goto again;
+		}
+
+		ret = aops->prepare_write(file, page, offset, offset+len);
+		if (ret) {
+			if (ret != AOP_TRUNCATED_PAGE)
+				unlock_page(page);
+			page_cache_release(page);
+			if (pos + len > inode->i_size)
+				vmtruncate(inode, inode->i_size);
+			if (ret == AOP_TRUNCATED_PAGE)
+				goto again;
+		}
+		return ret;
+	}
+}
+EXPORT_SYMBOL(pagecache_write_begin);
+
+int pagecache_write_end(struct file *file, struct address_space *mapping,
+				loff_t pos, unsigned len, unsigned copied,
+				struct page *page, void *fsdata)
+{
+	const struct address_space_operations *aops = mapping->a_ops;
+	int ret;
+
+	if (aops->write_end) {
+		mark_page_accessed(page);
+		ret = aops->write_end(file, mapping, pos, len, copied,
+							page, fsdata);
+	} else {
+		unsigned offset = pos & (PAGE_CACHE_SIZE - 1);
+		struct inode *inode = mapping->host;
+
+		flush_dcache_page(page);
+		ret = aops->commit_write(file, page, offset, offset+len);
+		unlock_page(page);
+		mark_page_accessed(page);
+		page_cache_release(page);
+		BUG_ON(ret == AOP_TRUNCATED_PAGE); /* can't deal with */
+
+		if (ret < 0) {
+			if (pos + len > inode->i_size)
+				vmtruncate(inode, inode->i_size);
+		} else if (ret > 0)
+			ret = min_t(size_t, copied, ret);
+		else
+			ret = copied;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(pagecache_write_end);
+
 ssize_t
 generic_file_direct_write(struct kiocb *iocb, const struct iovec *iov,
 		unsigned long *nr_segs, loff_t pos, loff_t *ppos,
@@ -1886,8 +1984,7 @@ EXPORT_SYMBOL(generic_file_direct_write);
  * Find or create a page at the given pagecache position. Return the locked
  * page. This function is specifically for buffered writes.
  */
-static struct page *__grab_cache_page(struct address_space *mapping,
-							pgoff_t index)
+struct page *__grab_cache_page(struct address_space *mapping, pgoff_t index)
 {
 	int status;
 	struct page *page;
@@ -1908,20 +2005,16 @@ repeat:
 	}
 	return page;
 }
+EXPORT_SYMBOL(__grab_cache_page);
 
-ssize_t
-generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
-		unsigned long nr_segs, loff_t pos, loff_t *ppos,
-		size_t count, ssize_t written)
+static ssize_t generic_perform_write_2copy(struct file *file,
+				struct iov_iter *i, loff_t pos)
 {
-	struct file *file = iocb->ki_filp;
 	struct address_space *mapping = file->f_mapping;
 	const struct address_space_operations *a_ops = mapping->a_ops;
-	struct inode 	*inode = mapping->host;
-	long		status = 0;
-	struct iov_iter i;
-
-	iov_iter_init(&i, iov, nr_segs, count, written);
+	struct inode *inode = mapping->host;
+	long status = 0;
+	ssize_t written = 0;
 
 	do {
 		struct page *src_page;
@@ -1934,7 +2027,7 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 		offset = (pos & (PAGE_CACHE_SIZE - 1));
 		index = pos >> PAGE_CACHE_SHIFT;
 		bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
-						iov_iter_count(&i));
+						iov_iter_count(i));
 
 		/*
 		 * a non-NULL src_page indicates that we're doing the
@@ -1952,7 +2045,7 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 		 * to check that the address is actually valid, when atomic
 		 * usercopies are used, below.
 		 */
-		if (unlikely(iov_iter_fault_in_readable(&i))) {
+		if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
 			status = -EFAULT;
 			break;
 		}
@@ -1983,7 +2076,7 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 			 * same reason as we can't take a page fault with a
 			 * page locked (as explained below).
 			 */
-			copied = iov_iter_copy_from_user(src_page, &i,
+			copied = iov_iter_copy_from_user(src_page, i,
 								offset, bytes);
 			if (unlikely(copied == 0)) {
 				status = -EFAULT;
@@ -2008,7 +2101,6 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 				page_cache_release(src_page);
 				continue;
 			}
-
 		}
 
 		status = a_ops->prepare_write(file, page, offset, offset+bytes);
@@ -2030,7 +2122,7 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 			 * really matter.
 			 */
 			pagefault_disable();
-			copied = iov_iter_copy_from_user_atomic(page, &i,
+			copied = iov_iter_copy_from_user_atomic(page, i,
 								offset, bytes);
 			pagefault_enable();
 		} else {
@@ -2056,9 +2148,9 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 		if (src_page)
 			page_cache_release(src_page);
 
-		iov_iter_advance(&i, copied);
-		written += copied;
+		iov_iter_advance(i, copied);
 		pos += copied;
+		written += copied;
 
 		balance_dirty_pages_ratelimited(mapping);
 		cond_resched();
@@ -2082,13 +2174,117 @@ fs_write_aop_error:
 			continue;
 		else
 			break;
-	} while (iov_iter_count(&i));
-	*ppos = pos;
+	} while (iov_iter_count(i));
+
+	return written ? written : status;
+}
+
+static ssize_t generic_perform_write(struct file *file,
+				struct iov_iter *i, loff_t pos)
+{
+	struct address_space *mapping = file->f_mapping;
+	const struct address_space_operations *a_ops = mapping->a_ops;
+	long status = 0;
+	ssize_t written = 0;
+
+	do {
+		struct page *page;
+		pgoff_t index;		/* Pagecache index for current page */
+		unsigned long offset;	/* Offset into pagecache page */
+		unsigned long bytes;	/* Bytes to write to page */
+		size_t copied;		/* Bytes copied from user */
+		void *fsdata;
+
+		offset = (pos & (PAGE_CACHE_SIZE - 1));
+		index = pos >> PAGE_CACHE_SHIFT;
+		bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
+						iov_iter_count(i));
+
+again:
+
+		/*
+		 * Bring in the user page that we will copy from _first_.
+		 * Otherwise there's a nasty deadlock on copying from the
+		 * same page as we're writing to, without it being marked
+		 * up-to-date.
+		 *
+		 * Not only is this an optimisation, but it is also required
+		 * to check that the address is actually valid, when atomic
+		 * usercopies are used, below.
+		 */
+		if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
+			status = -EFAULT;
+			break;
+		}
+
+		status = a_ops->write_begin(file, mapping, pos, bytes, 0,
+						&page, &fsdata);
+		if (unlikely(status))
+			break;
+
+		pagefault_disable();
+		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
+		pagefault_enable();
+		flush_dcache_page(page);
+
+		status = a_ops->write_end(file, mapping, pos, bytes, copied,
+						page, fsdata);
+		if (unlikely(status < 0))
+			break;
+		copied = status;
+
+		cond_resched();
+
+		if (unlikely(copied == 0)) {
+			/*
+			 * If we were unable to copy any data at all, we must
+			 * fall back to a single segment length write.
+			 *
+			 * If we didn't fallback here, we could livelock
+			 * because not all segments in the iov can be copied at
+			 * once without a pagefault.
+			 */
+			bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
+						iov_iter_single_seg_count(i));
+			goto again;
+		}
+		iov_iter_advance(i, copied);
+		pos += copied;
+		written += copied;
+
+		balance_dirty_pages_ratelimited(mapping);
+
+	} while (iov_iter_count(i));
+
+	return written ? written : status;
+}
+
+ssize_t
+generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
+		unsigned long nr_segs, loff_t pos, loff_t *ppos,
+		size_t count, ssize_t written)
+{
+	struct file *file = iocb->ki_filp;
+	struct address_space *mapping = file->f_mapping;
+	const struct address_space_operations *a_ops = mapping->a_ops;
+	struct inode *inode = mapping->host;
+	ssize_t status;
+	struct iov_iter i;
+
+	iov_iter_init(&i, iov, nr_segs, count, written);
+	if (a_ops->write_begin)
+		status = generic_perform_write(file, &i, pos);
+	else
+		status = generic_perform_write_2copy(file, &i, pos);
 
-	/*
-	 * For now, when the user asks for O_SYNC, we'll actually give O_DSYNC
-	 */
 	if (likely(status >= 0)) {
+		written += status;
+		*ppos = pos + status;
+
+		/*
+		 * For now, when the user asks for O_SYNC, we'll actually give
+		 * O_DSYNC
+		 */
 		if (unlikely((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
 			if (!a_ops->writepage || !is_sync_kiocb(iocb))
 				status = generic_osync_inode(inode, mapping,