Re: [RFC PATCH 12/18] iomap: don't increase i_size if it's not a write operation

[Christoph Hellwig <hch@infradead.org>]

On Thu, Nov 23, 2023 at 08:51:14PM +0800, Zhang Yi wrote:
> index fd4d43bafd1b..3b9ba390dd1b 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -852,13 +852,13 @@ static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
>  	 * 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) {
> +	if ((iter->flags & IOMAP_WRITE) && 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)
> +	if ((iter->flags & IOMAP_WRITE) && old_size < pos)
>  		pagecache_isize_extended(iter->inode, old_size, pos);
>  	if (ret < len)
>  		iomap_write_failed(iter->inode, pos + ret, len - ret);

I agree with your rationale, but I hate how this code ends up
looking.  In many ways iomap_write_end seems like the wrong
place to update the inode size anyway.  I've not done a deep
analysis, but I think there shouldn't really be any major blocker
to only setting IOMAP_F_SIZE_CHANGED in iomap_write_end, and then
move updating i_size and calling pagecache_isize_extended to
iomap_write_iter.