Re: [PATCH 3/4] iomap: don't increase i_size if it's not a write operation

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

On Tue, Mar 12, 2024 at 08:59:15PM +0800, Zhang Yi wrote:
> On 2024/3/11 23:48, Darrick J. Wong wrote:
> > On Mon, Mar 11, 2024 at 08:22:54PM +0800, Zhang Yi wrote:
> >> From: Zhang Yi <yi.zhang@huawei.com>
> >>
> >> Increase i_size in iomap_zero_range() and iomap_unshare_iter() is not
> >> needed, the caller should handle it. Especially, when truncate partial
> >> block, we could not increase i_size beyond the new EOF here. It doesn't
> >> affect xfs and gfs2 now because they set the new file size after zero
> >> out, it doesn't matter that a transient increase in i_size, but it will
> >> affect ext4 because it set file size before truncate.
> > 
> >>                                                       At the same time,
> >> iomap_write_failed() is also not needed for above two cases too, so
> >> factor them out and move them to iomap_write_iter() and
> >> iomap_zero_iter().
> > 
> > This change should be a separate patch with its own justification.
> > Which is, AFAICT, something along the lines of:
> > 
> > "Unsharing and zeroing can only happen within EOF, so there is never a
> > need to perform posteof pagecache truncation if write begin fails."
> 
> Sure.
> 
> > 
> >> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> > 
> > Doesn't this patch fix a bug in ext4?
> 
> Yeah, the same as Christoph answered.
> 
> > 
> >> ---
> >>  fs/iomap/buffered-io.c | 59 +++++++++++++++++++++---------------------
> >>  1 file changed, 30 insertions(+), 29 deletions(-)
> >>
> >> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> >> index 093c4515b22a..19f91324c690 100644
> >> --- a/fs/iomap/buffered-io.c
> >> +++ b/fs/iomap/buffered-io.c
> >> @@ -786,7 +786,6 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
> >>  
> >>  out_unlock:
> >>  	__iomap_put_folio(iter, pos, 0, folio);
> >> -	iomap_write_failed(iter->inode, pos, len);
> >>  
> >>  	return status;
> >>  }
> >> @@ -838,34 +837,13 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
> >>  		size_t copied, struct folio *folio)
> >>  {
> >>  	const struct iomap *srcmap = iomap_iter_srcmap(iter);
> >> -	loff_t old_size = iter->inode->i_size;
> >> -	size_t ret;
> >> -
> >> -	if (srcmap->type == IOMAP_INLINE) {
> >> -		ret = iomap_write_end_inline(iter, folio, pos, copied);
> >> -	} else if (srcmap->flags & IOMAP_F_BUFFER_HEAD) {
> >> -		ret = block_write_end(NULL, iter->inode->i_mapping, pos, len,
> >> -				copied, &folio->page, NULL);
> >> -	} else {
> >> -		ret = __iomap_write_end(iter->inode, pos, len, copied, folio);
> >> -	}
> >>  
> >> -	/*
> >> -	 * Update the in-memory inode size after copying the data into the page
> >> -	 * cache.  It's up to the file system to write the updated size to disk,
> >> -	 * preferably after I/O completion so that no stale data is exposed.
> >> -	 */
> >> -	if (pos + ret > old_size) {
> >> -		i_size_write(iter->inode, pos + ret);
> >> -		iter->iomap.flags |= IOMAP_F_SIZE_CHANGED;
> >> -	}
> >> -	__iomap_put_folio(iter, pos, ret, folio);
> >> -
> >> -	if (old_size < pos)
> >> -		pagecache_isize_extended(iter->inode, old_size, pos);
> >> -	if (ret < len)
> >> -		iomap_write_failed(iter->inode, pos + ret, len - ret);
> >> -	return ret;
> >> +	if (srcmap->type == IOMAP_INLINE)
> >> +		return iomap_write_end_inline(iter, folio, pos, copied);
> >> +	if (srcmap->flags & IOMAP_F_BUFFER_HEAD)
> >> +		return block_write_end(NULL, iter->inode->i_mapping, pos, len,
> >> +				       copied, &folio->page, NULL);
> >> +	return __iomap_write_end(iter->inode, pos, len, copied, folio);
> >>  }
> >>  
> >>  static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >> @@ -880,6 +858,7 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >>  
> >>  	do {
> >>  		struct folio *folio;
> >> +		loff_t old_size;
> >>  		size_t offset;		/* Offset into folio */
> >>  		size_t bytes;		/* Bytes to write to folio */
> >>  		size_t copied;		/* Bytes copied from user */
> >> @@ -912,8 +891,10 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >>  		}
> >>  
> >>  		status = iomap_write_begin(iter, pos, bytes, &folio);
> >> -		if (unlikely(status))
> >> +		if (unlikely(status)) {
> >> +			iomap_write_failed(iter->inode, pos, bytes);
> >>  			break;
> >> +		}
> >>  		if (iter->iomap.flags & IOMAP_F_STALE)
> >>  			break;
> >>  
> >> @@ -927,6 +908,24 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >>  		copied = copy_folio_from_iter_atomic(folio, offset, bytes, i);
> >>  		status = iomap_write_end(iter, pos, bytes, copied, folio);
> >>  
> >> +		/*
> >> +		 * Update the in-memory inode size after copying the data into
> >> +		 * the page cache.  It's up to the file system to write the
> >> +		 * updated size to disk, preferably after I/O completion so that
> >> +		 * no stale data is exposed.
> >> +		 */
> >> +		old_size = iter->inode->i_size;
> >> +		if (pos + status > old_size) {
> >> +			i_size_write(iter->inode, pos + status);
> >> +			iter->iomap.flags |= IOMAP_F_SIZE_CHANGED;
> >> +		}
> >> +		__iomap_put_folio(iter, pos, status, folio);
> > 
> > Why is it necessary to hoist the __iomap_put_folio calls from
> > iomap_write_end into iomap_write_iter, iomap_unshare_iter, and
> > iomap_zero_iter?  None of those functions seem to use it, and it makes
> > more sense to me that iomap_write_end releases the folio that
> > iomap_write_begin returned.
> > 
> 
> Because we have to update i_size before __iomap_put_folio() in
> iomap_write_iter(). If not, once we unlock folio, it could be raced
> by the backgroud write back which could start writing back and call
> folio_zero_segment() (please see iomap_writepage_handle_eof()) to
> zero out the valid data beyond the not updated i_size. So we
> have to move out __iomap_put_folio() out together with the i_size
> updating.

Ahah.  Please make a note of that in the comment for dunces like me.

	/*
	 * Update the in-memory inode size after copying the data into
	 * the page cache.  It's up to the file system to write the
	 * updated size to disk, preferably after I/O completion so that
	 * no stale data is exposed.  Only once that's done can we
	 * unlock and release the folio.
	 */

--D

> Thanks,
> Yi.
> 
>