nommu: ramfs: pages allocated to an inode's pagecache may get wrongly discarded

The pages attached to a ramfs inode's pagecache by truncation from nothing
- as done by SYSV SHM for example - may get discarded under memory
pressure.

The problem is that the pages are not marked dirty.  Anything that creates
data in an MMU-based ramfs will cause the pages holding that data will
cause the set_page_dirty() aop to be called.

For the NOMMU-based mmap, set_page_dirty() may be called by write(), but
it won't be called by page-writing faults on writable mmaps, and it isn't
called by ramfs_nommu_expand_for_mapping() when a file is being truncated
from nothing to allocate a contiguous run.

The solution is to mark the pages dirty at the point of allocation by the
truncation code.

Signed-off-by: Enrik Berkhan <Enrik.Berkhan@ge.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ramfs/file-nommu.c

@@ -114,6 +114,9 @@ int ramfs_nommu_expand_for_mapping(struct inode *inode, size_t newsize)
 		if (!pagevec_add(&lru_pvec, page))
 			__pagevec_lru_add_file(&lru_pvec);
 
+		/* prevent the page from being discarded on memory pressure */
+		SetPageDirty(page);
+
 		unlock_page(page);
 	}