linux-kernel - [PATCH] rd: Preserve the dirty bit in init_page

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Message-ID: <m14pgsknmd.fsf_-_@ebiederm.dsl.xmission.com>
Date:	Mon, 15 Oct 2007 16:40:42 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: [PATCH] rd: Preserve the dirty bit in init_page_buffers()

The problem:  When we are trying to free buffers try_to_free_buffers()
will look at ramdisk pages with clean buffer heads and remove the
dirty bit from the page.  Resulting in ramdisk pages with data that get
removed from the page cache.  Ouch!

Buffer heads appear on ramdisk pages when a filesystem calls
__getblk() which through a series of function calls eventually calls
init_page_buffers().

So to fix the mismatch between buffer head and page state this patch
modifies init_page_buffers() to transfer the dirty bit from the page to
the buffer heads like we currently do for the uptodate bit.

This patch is safe as only __getblk calls init_page_buffers, and
there are only two implementations of block devices cached in the
page cache.  The generic implementation in block_dev.c and the
implementation in rd.c.

The generic implementation of block devices always does everything
in terms of buffer heads so it always has buffer heads allocated
before a page is marked dirty so this change does not affect it.

Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
---
 fs/buffer.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 75b51df..8b87beb 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -972,6 +972,7 @@ init_page_buffers(struct page *page, struct block_device *bdev,
 	struct buffer_head *head = page_buffers(page);
 	struct buffer_head *bh = head;
 	int uptodate = PageUptodate(page);
+	int dirty = PageDirty(page);

 	do {
 		if (!buffer_mapped(bh)) {
@@ -980,6 +981,8 @@ init_page_buffers(struct page *page, struct block_device *bdev,
 			bh->b_blocknr = block;
 			if (uptodate)
 				set_buffer_uptodate(bh);
+			if (dirty)
+				set_buffer_dirty(bh);
 			set_buffer_mapped(bh);
 		}
 		block++;
-- 
1.5.3.rc6.17.g1911

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