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

[Christoph Hellwig]

On Mon, Jun 07, 2021 at 07:55:22PM +0800, Coly Li wrote:
> On 6/7/21 7:06 PM, Christoph Hellwig wrote:
> > 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
> 
> It was suggested to not directly access BIO_MAX_VECS by you, maybe I
> misunderstood you.

Yes, drivers really should not care about it.  But hiding that behind
a tiny wrapper doesn't help either.

> > 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.
> 
> 
> It is just because bch_bio_map() assume the maximum bio size is 1MB. It
> was not true since the multiple pages bvecs
> was merged in mainline kernel.
>  
> The PAGE_SECTORS part is legacy for 1MB maximum size bio (256*4KB), it
> should be fixed/improved later to
> use multiple pages for bio size > 1MB and replace bch_bio_map().

bch_bio_map and bch_bio_alloc_pages that poke directly into the bio are
the root cause of a lot of these problems.

I had a series fixing some of that but Kent did not like it.  Drivers must
not diretly access bi_vcnt or directly build bios.

> Not any more. Now the line limit is 100 characters. Though I still
> prefer 80 characters, place 86 characters in single line
> makes the change more obvious.

It makes it really hard to read in a normal terminal.