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

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Date:	Tue, 29 Mar 2016 14:18:33 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Shaohua Li <shli@...com>
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 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.