linux-kernel - Re: [PATCH 1/3] Preserve the dirty bit in init_page

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Message-ID: <465A55E7.50904@yahoo.com.au>
Date:	Mon, 28 May 2007 14:09:11 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] Preserve the dirty bit in init_page_buffers

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 <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 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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