Re: [rfc] weirdness in bio_map_user_iov()

From: Sagi Grimberg
Date: Fri Sep 23 2016 - 17:48:01 EST

Hey Al,

What happens if we feed it a 3-element iovec array, one page in each?
AFAICS, bio_add_pc_page() is called for each of those pages, even if
the previous calls have failed - break is only out of the inner loop.

Sure, failure due to exceeded request size means that everything after
that one will fail, but what of e.g.
* If the queue doesn't support SG gaps and adding this
* offset would create a gap, disallow it.
if (bvec_gap_to_prev(q, prev, offset))
return 0;
in there? Won't we risk having the first and the third pages added, with
the second one quietly skipped? Jens, looks like it had come from you
(by way of jejb). Am I missing something subtle here?

So AFAICT, 'gappy' iovecs will never reach bio_map_user_iov() because it
is checked before in blk_rq_map_user_iov():

if (map_data)
copy = true;
else if (iov_iter_alignment(iter) & align)
copy = true;
else if (queue_virt_boundary(q))
copy = queue_virt_boundary(q) & iov_iter_gap_alignment(iter);

if iov_iter_gap_alignment(iter) detects gaps it will use
a nice aligned bounce via bio_copy_user_iov().