linux-kernel - Re: [PATCH] block: don't make BLK_DEF_MAX

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Message-ID: <20160329220053.GA1716708@devbig084.prn1.facebook.com>
Date:	Tue, 29 Mar 2016 15:01:10 -0700
From:	Shaohua Li <shli@...com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	<linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<axboe@...com>, <Kernel-team@...com>,
	"4.2+" <stable@...r.kernel.org>, Ming Lei <ming.lei@...onical.com>
Subject: Re: [PATCH] block: don't make BLK_DEF_MAX_SECTORS too big

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.