linux-kernel - [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

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Message-ID: <20090311153034.9389.19938.stgit@warthog.procyon.org.uk>
Date:	Wed, 11 Mar 2009 15:30:35 +0000
From:	David Howells <dhowells@...hat.com>
To:	torvalds@...l.org, akpm@...ux-foundation.org, peterz@...radead.org
Cc:	Enrik.Berkhan@...com, dhowells@...hat.com, uclinux-dev@...inux.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get
	wrongly discarded

From: Enrik Berkhan <Enrik.Berkhan@...com>

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@...com>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/ramfs/file-nommu.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/fs/ramfs/file-nommu.c b/fs/ramfs/file-nommu.c
index b9b567a..90d72be 100644
--- a/fs/ramfs/file-nommu.c
+++ b/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);
 	}

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