Re: [PATCH] iomap: skip unnecessary ifs_block_is_uptodate check

[Darrick J. Wong]

On Wed, Apr 09, 2025 at 01:29:24AM +0800, Gou Hao wrote:
> After the first 'for' loop, the first call to
> ifs_block_is_uptodate always evaluates to 0.
> 
> Signed-off-by: Gou Hao <gouhao@xxxxxxxxxxxxx>
> ---
>  fs/iomap/buffered-io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 31553372b33a..2f52e8e61240 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -259,7 +259,7 @@ static void iomap_adjust_read_range(struct inode *inode, struct folio *folio,
>  		}
>  
>  		/* truncate len if we find any trailing uptodate block(s) */
> -		for ( ; i <= last; i++) {
> +		for (i++; i <= last; i++) {

Hmmm... prior to the loop, $i is either the first !uptodate block, or
it's past $last.  Assuming there's no overflow (there's no combination
of huge folios and tiny blksize that I can think of) then yeah, there's
no point in retesting that the same block $i is uptodate since we hold
the folio lock so nobody else could have set uptodate.

Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>

--D


>  			if (ifs_block_is_uptodate(ifs, i)) {
>  				plen -= (last - i + 1) * block_size;
>  				last = i - 1;
> -- 
> 2.20.1
> 
>