Re: [PATCH 1/3] Preserve the dirty bit in init_page_buffers

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Eric W. Biederman wrote:
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 state 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.

Ouch indeed!

But can we ever have a dirty page at init_page_buffers-time?

I would have thought we can fix this simply by removing the
broken ramdisk_set_page_dirty (as far as the comment goes, we
set CAP_NO_ACCT_DIRTY anyway, so the normal set_page_dirty
should handle everything properly, no?).


Signed-off-by: Eric W. Biederman <[email protected]>
---
 fs/buffer.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index aa68206..c6b58e8 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -953,6 +953,7 @@ 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)) {
@@ -961,6 +962,8 @@ 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++;


--
SUSE Labs, Novell Inc.
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