linux-kernel - Re: [PATCH v2] block: bio_map_user_iov should not be limited to BIO_MAX

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Date:   Thu, 18 Apr 2019 17:29:19 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, stefanha@...hat.com,
        stable@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>,
        "open list:BLOCK LAYER" <linux-block@...r.kernel.org>
Subject: Re: [PATCH v2] block: bio_map_user_iov should not be limited to
 BIO_MAX_PAGES

On Thu, Apr 18, 2019 at 10:42:21AM +0200, Paolo Bonzini wrote:
> On 18/04/19 04:19, Ming Lei wrote:
> > Hi Paolo,
> > 
> > On Wed, Apr 17, 2019 at 01:52:07PM +0200, Paolo Bonzini wrote:
> >> Because bio_kmalloc uses inline iovecs, the limit on the number of entries
> >> is not BIO_MAX_PAGES but rather UIO_MAXIOV, which indeed is already checked
> >> in bio_kmalloc.  This could cause SG_IO requests to be truncated and the HBA
> >> to report a DMA overrun.
> > 
> > BIO_MAX_PAGES only limits the single bio's max vector number, if one bio
> > can't hold all user space request, new bio will be allocated and appended
> > to the passthrough request if queue limits aren't reached.
> 
> Stupid question: where?  I don't see any place starting at
> blk_rq_map_user_iov (and then __blk_rq_map_user_iov->bio_map_user_iov)
> that would allocate a second bio.  The only bio_kmalloc in that path is
> the one I'm patching.

Each bio is created inside __blk_rq_map_user_iov() which is run inside
a loop, and the created bio is added to request via blk_rq_append_bio(),
see the following code:

blk_rq_map_user_iov
	__blk_rq_map_user_iov
		blk_rq_append_bio

blk_rq_map_user_iov():
		...
        do {
                ret =__blk_rq_map_user_iov(rq, map_data, &i, gfp_mask, copy);
                if (ret)
                        goto unmap_rq;
                if (!bio)
                        bio = rq->bio;
        } while (iov_iter_count(&i));
		...

Thanks,
Ming