Re: [PATCH 1/2] btrfs: consume the given iter directly instead of copying in csum_one_bio()

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2026/9/3 02:21, Daniel Vacek 写道:
> This is just a small cleanup to avoid copying the iter twice in async case.
> We already have a copy csum_one_bio() can consume directly. No need to copy
> again the second time.
> In the sync case we can copy in the caller (as we did before dd57c78aec39
> ("btrfs: introduce btrfs_bio::async_csum")).
> 
> Signed-off-by: Daniel Vacek <neelx@suse.com>
> ---
>   fs/btrfs/file-item.c | 14 +++++++-------
>   1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
> index 581ca5653be9..5a5cffb18922 100644
> --- a/fs/btrfs/file-item.c
> +++ b/fs/btrfs/file-item.c
> @@ -797,18 +797,16 @@ int btrfs_lookup_csums_bitmap(struct btrfs_root *root, struct btrfs_path *path,
>   	return ret;
>   }
>   
> -static void csum_one_bio(struct btrfs_bio *bbio, struct bvec_iter *src)
> +static void csum_one_bio(struct btrfs_bio *bbio, struct bvec_iter *iter)

Since we're here, what about adding const prefix to iter?

>   {
>   	struct btrfs_inode *inode = bbio->inode;
>   	struct btrfs_fs_info *fs_info = inode->root->fs_info;
>   	struct btrfs_ordered_sum *sums = bbio->sums;
> -	struct bvec_iter iter;
>   	const u32 blocksize = fs_info->sectorsize;
>   	int index = 0;
>   
> -	for (iter = *src; iter.bi_size; bio_advance_iter(&bbio->bio, &iter, blocksize)) {

I'm thinking the opposite way.

What about still keeping a local bvec_iter copy, but remove 
csum_saved_iter completely?

Since at bio calculation time, the bio is not yet submitted, so the 
csum_saved_iter always match bio.bi_iter.

I think removing csum_saved_iter would also reduce the size of btrfs_bio.

Thanks,
Qu

> -		btrfs_csum_one_bio_block(fs_info, &bbio->bio, &iter,
> -					 sums->sums + index);
> +	for (; iter->bi_size; bio_advance_iter(&bbio->bio, iter, blocksize)) {
> +		btrfs_csum_one_bio_block(fs_info, &bbio->bio, iter, sums->sums + index);
>   
>   		index += fs_info->csum_size;
>   	}
> @@ -851,12 +849,14 @@ int btrfs_csum_one_bio(struct btrfs_bio *bbio, bool async)
>   	btrfs_add_ordered_sum(ordered, sums);
>   
>   	if (!async) {
> -		csum_one_bio(bbio, &bbio->bio.bi_iter);
> +		struct bvec_iter iter = bio->bi_iter;
> +
> +		csum_one_bio(bbio, &iter);
>   		return 0;
>   	}
>   	init_completion(&bbio->csum_done);
>   	bbio->async_csum = true;
> -	bbio->csum_saved_iter = bbio->bio.bi_iter;
> +	bbio->csum_saved_iter = bio->bi_iter;
>   	INIT_WORK(&bbio->csum_work, csum_one_bio_work);
>   	schedule_work(&bbio->csum_work);
>   	return 0;