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

[Coly Li]

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.


> 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().


>> +	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.  

Yes, they are in sectors, this is the maximum permitted bio size for 1MB
bio size. Now we can have bio > 1MB, and you modify
bio_alloc_bioset() parameter 'nr_iovecs from unsigned int to unsigned
short, so bio-size/page-size can be > 256 and overflow,
e.g. 258 overflows to 2, then the BUG in biovec_slab() is triggered.

I feel this is a long time existing issue in bcache code, and should be
fixed from bcache side, and your change helps to trigger
the problem explicitly.

>> -	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.

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.

Thanks.

Coly Li