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

[Zhang Yi <yi.zhang@huaweicloud.com>]

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.

Thanks,
Yi.