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

Nick Piggin
2007-05-28T04:09:30Z

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@xmission.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++;

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