Re: [PATCH] cifs: fix writeback race with file that is growing

[Suresh Jayaraman <sjayaraman-IBi9RG/b67k@public.gmane.org>]

On 11/26/2012 09:28 PM, Jeff Layton wrote:
> Commit eddb079deb4 created a regression in the writepages codepath.
> Previously, whenever it needed to check the size of the file, it did so
> by consulting the inode->i_size field directly. With that patch, the
> i_size was fetched once on entry into the writepages code and that value
> was used henceforth.
> 
> If the file is changing size though (for instance, if someone is writing
> to it or has truncated it), then that value is likely to be wrong. This
> can lead to data corruption. Pages past the EOF at the time that the
> writepages call was issued may be silently dropped and ignored because
> cifs_writepages wrongly assumes that the file must have been truncated
> in the interim.
> 
> Fix cifs_writepages to properly fetch the size from the inode->i_size
> field instead to properly account for this possibility.
> 
> Reported-and-Tested-by: Maxim Britov <ungifted01-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Signed-off-by: Jeff Layton <jlayton-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> ---
>  fs/cifs/file.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> index edb25b4..70b6f4c 100644
> --- a/fs/cifs/file.c
> +++ b/fs/cifs/file.c
> @@ -1794,7 +1794,6 @@ static int cifs_writepages(struct address_space *mapping,
>  	struct TCP_Server_Info *server;
>  	struct page *page;
>  	int rc = 0;
> -	loff_t isize = i_size_read(mapping->host);
>  
>  	/*
>  	 * If wsize is smaller than the page cache size, default to writing
> @@ -1899,7 +1898,7 @@ retry:
>  			 */
>  			set_page_writeback(page);
>  
> -			if (page_offset(page) >= isize) {
> +			if (page_offset(page) >= i_size_read(mapping->host)) {
>  				done = true;
>  				unlock_page(page);
>  				end_page_writeback(page);
> @@ -1932,7 +1931,8 @@ retry:
>  		wdata->offset = page_offset(wdata->pages[0]);
>  		wdata->pagesz = PAGE_CACHE_SIZE;
>  		wdata->tailsz =
> -			min(isize - page_offset(wdata->pages[nr_pages - 1]),
> +			min(i_size_read(mapping->host) -
> +			    page_offset(wdata->pages[nr_pages - 1]),
>  			    (loff_t)PAGE_CACHE_SIZE);

Good catch. Looks correct to me. Wondering whether we would need a
similar fix in cifs_write_begin() where we get the i_size and then
immediately do check whether the page lies beyond EOF?


-- 
Suresh Jayaraman