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SubjectRe: [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

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