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Date:   Tue, 17 Aug 2021 06:06:12 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Guoqing Jiang <guoqing.jiang@linux.dev>, song@kernel.org,
        linux-raid@vger.kernel.org, jens@chianterastutte.eu,
        linux-block@vger.kernel.org
Subject: Re: [PATCH] raid1: ensure bio doesn't have more than BIO_MAX_VECS
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On Mon, Aug 16, 2021 at 07:40:48PM +0800, Ming Lei wrote:
> > > 
> > > 0 ~ 254: each bvec's length is 512
> > > 255: bvec's length is 8192
> > > 
> > > the total length is just 512*255 + 8192 = 138752 bytes = 271 sectors, but it
> > > still may need 257 bvecs, which can't be allocated via bio_alloc_bioset().
> > 
> > Yes, we still need the rounding magic that alloc_behind_master_bio uses
> > here.
> 
> But it is wrong to use max sectors to limit number of bvecs(segments), isn't it?

The raid1 write behind code cares about the size ofa bio it can reach by
adding order 0 pages to it.  The bvecs are part of that and I think the
calculation in the patch documents that a well.