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 11:36:03 +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 11:29, Ming Lei wrote:
> 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:

Uff, I can't read apparently. :(  This is the commit that introduced it:

    commit 4d6af73d9e43f78651a43ee4c5ad221107ac8365
    Author: Christoph Hellwig <hch@....de>
    Date:   Wed Mar 2 18:07:14 2016 +0100

    block: support large requests in blk_rq_map_user_iov
    
    This patch adds support for larger requests in blk_rq_map_user_iov by
    allowing it to build multiple bios for a request.  This functionality
    used to exist for the non-vectored blk_rq_map_user in the past, and
    this patch reuses the existing functionality for it on the unmap side,
    which stuck around.  Thanks to the iov_iter API supporting multiple
    bios is fairly trivial, as we can just iterate the iov until we've
    consumed the whole iov_iter.
    
    Signed-off-by: Christoph Hellwig <hch@....de>
    Reported-by: Jeff Lien <Jeff.Lien@...t.com>
    Tested-by: Jeff Lien <Jeff.Lien@...t.com>
    Reviewed-by: Keith Busch <keith.busch@...el.com>
    Signed-off-by: Jens Axboe <axboe@...com>

Paolo