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

[Ming Lei]

On Thu, Apr 18, 2019 at 10:19:04AM +0800, 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.
> 
> 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.

Also we have enabled multipage bvec for passthough IO[1], we shouldn't
need to allocate so big max io vectors any more, and actually the reasonable
number is (PAGE_SIZE - sizeof(struct bio)) / sizeof(struct bio_vec), then we
can avoid to stress mm for high order allocation.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/commit/?h=for-5.2/block&id=489fbbcb51d0249569d863f9220de69cb31f1922

Thanks,
Ming