Re: [PATCH v3 4/4] iomap: warn on zero range of a post-eof folio

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Nov 08, 2024 at 07:42:46AM -0500, Brian Foster wrote:
> iomap_zero_range() uses buffered writes for manual zeroing, no
> longer updates i_size for such writes, but is still explicitly
> called for post-eof ranges. The historical use case for this is
> zeroing post-eof speculative preallocation on extending writes from
> XFS. However, XFS also recently changed to convert all post-eof
> delalloc mappings to unwritten in the iomap_begin() handler, which
> means it now never expects manual zeroing of post-eof mappings. In
> other words, all post-eof mappings should be reported as holes or
> unwritten.
> 
> This is a subtle dependency that can be hard to detect if violated
> because associated codepaths are likely to update i_size after folio
> locks are dropped, but before writeback happens to occur. For
> example, if XFS reverts back to some form of manual zeroing of
> post-eof blocks on write extension, writeback of those zeroed folios
> will now race with the presumed i_size update from the subsequent
> buffered write.
> 
> Since iomap_zero_range() can't correctly zero post-eof mappings
> beyond EOF without updating i_size, warn if this ever occurs. This
> serves as minimal indication that if this use case is reintroduced
> by a filesystem, iomap_zero_range() might need to reconsider i_size
> updates for write extending use cases.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/iomap/buffered-io.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 7f40234a301e..e18830e4809b 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1354,6 +1354,7 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
>  {
>  	loff_t pos = iter->pos;
>  	loff_t length = iomap_length(iter);
> +	loff_t isize = iter->inode->i_size;
>  	loff_t written = 0;
>  
>  	do {
> @@ -1369,6 +1370,8 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
>  		if (iter->iomap.flags & IOMAP_F_STALE)
>  			break;
>  
> +		/* warn about zeroing folios beyond eof that won't write back */
> +		WARN_ON_ONCE(folio_pos(folio) > isize);

		WARN_ON_ONCE(folio_pos(folio) > iter->inode->i_size));?

No need to have the extra local variable for something that shouldn't
ever happen.  Do you need i_size_read for correctness here?

--D

>  		offset = offset_in_folio(folio, pos);
>  		if (bytes > folio_size(folio) - offset)
>  			bytes = folio_size(folio) - offset;
> -- 
> 2.47.0
> 
>