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 10:42:21 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ming Lei <ming.lei@...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 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.

> So I understand SG_IO request shouldn't be truncated because of
> BIO_MAX_PAGES, or could you explain it in a bit detail or provide
> a reproducer?

Unfortunately I don't have a reproducer.  Actually any userspace SG_IO
above 4 MB triggers it, but you need the right HBA to ensure that it
reaches bio_kmalloc.

Paolo