Re: [PATCH] block: don't make BLK_DEF_MAX_SECTORS too big

[Shaohua Li <shli@fb.com>]

On Tue, Mar 29, 2016 at 02:18:33PM -0700, Christoph Hellwig wrote:
> On Tue, Mar 29, 2016 at 09:42:33AM -0700, Shaohua Li wrote:
> > bio_alloc_bioset() allocates bvecs from bvec_slabs which can only
> > allocate maximum 256 bvec (eg, 1M for 4k pages). We can't bump
> > BLK_DEF_MAX_SECTORS to exceed this value otherwise bio_alloc_bioset will
> > fail.
> 
> This might be true, but it's not a good enough reason.  Request based
> driver couldn't care less about the limits of bio_alloc_bioset.
> 
> It seems the bug is that somone (would be great to know whoe exactly)
> passes a too large value to bio_alloc_bioset. And given that we still
> have bio_add_page around for actually adding pages to a bio it seems
> like the proper fix would be to simply clamp down the actual allocation
> and segment limit inside bio_alloc_bioset.  Which would also help to
> eventually remove code doing just that in tons of callers.

The problem is bcache allocates a big bio (with bio_alloc). The bio is
split with blk_queue_split, but it isn't split to small size because
queue limit. the bio is cloned later in md, which uses bio_alloc_bioset.
bio_alloc_bioset itself can't allocate big size bio.