Re: [PATCH v2] block: bio_map_user_iov should not be limited to BIO_MAX_PAGES

[Ming Lei <ming.lei@redhat.com>]

On Thu, Apr 18, 2019 at 11:36:03AM +0200, Paolo Bonzini wrote:
> 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@lst.de>
>     Date:   Wed Mar 2 18:07:14 2016 +0100
> 
>     block: support large requests in blk_rq_map_user_iov

Exactly, the above commit starts to build multiple bios for a request.

Then I guess your issue is triggered on kernel without the commit.

Thanks,
Ming