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

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?

>
> Note that if the argument to iov_iter_npages were changed to UIO_MAXIOV,
> we would still truncate SG_IO requests beyond UIO_MAXIOV pages. Changing
> it to UIO_MAXIOV + 1 instead ensures that bio_kmalloc notices that the
> request is too big and blocks it.

We should pass UIO_MAXIOV instead of UIO_MAXIOV + 1, otherwise bio_kmalloc()
will fail. Otherwise, the patch looks fine, but shouldn't be a fix if my
above analysis is correct.

Thanks,
Ming

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