linux-kernel - Re: [rfc] weirdness in bio_map_user

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Date:   Fri, 23 Sep 2016 14:47:55 -0700
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Al Viro <viro@...IV.linux.org.uk>, Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [rfc] weirdness in bio_map_user_iov()

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().