Re: [PATCH v5 2/2] bcache: avoid oversized read request in cache missing code path

[Christoph Hellwig]

On Mon, Jun 07, 2021 at 06:35:39PM +0800, Coly Li wrote:
> +	/* Limitation for valid replace key size and cache_bio bvecs number */
> +	size_limit = min_t(unsigned int, bio_max_segs(UINT_MAX) * PAGE_SECTORS,
> +			   (1 << KEY_SIZE_BITS) - 1);

bio_max_segs kaps the argument to BIO_MAX_VECS, so you might as well
directly write BIO_MAX_VECS.  Can you explain the PAGE_SECTORS here a bit
more? Does this code path use discontiguous per-sector allocations?
Preferably in a comment.

> +	s->insert_bio_sectors = min3(size_limit, sectors, bio_sectors(bio));

Also I don't really understand the units involved here.
s->insert_bio_sectors, sectors, and bio_sectors is in unit of 512 byte
sectors.  

> -	miss = bio_next_split(bio, sectors, GFP_NOIO, &s->d->bio_split);
> +	miss = bio_next_split(bio, s->insert_bio_sectors, GFP_NOIO, &s->d->bio_split);

Overly long line.