netdev - skb_splice_bits() and large chunks in pipe (was Re: xfs_file_splice_read: possible circular locking dependency detected

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Message-ID: <20160918193112.GF2356@ZenIV.linux.org.uk>
Date:   Sun, 18 Sep 2016 20:31:12 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Nick Piggin <npiggin@...il.com>,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>
Subject: skb_splice_bits() and large chunks in pipe (was Re:
 xfs_file_splice_read: possible circular locking dependency detected

FWIW, I'm not sure if skb_splice_bits() can't land us in trouble; fragments
might come from compound pages and I'm not entirely convinced that we won't
end up with coalesced fragments putting more than PAGE_SIZE into a single
pipe_buffer.  And that could badly confuse a bunch of code.

Can that legitimately happen?  If so, we'll need to audit quite a few
->splice_write()-related codepaths; FUSE, in particular, is very likely
to be unhappy with that kind of stuff, and it's not the only place where
we might count upon never seeing e.g. longer than PAGE_SIZE chunks in
bio_vec.  It shouldn't be all that hard to fix, but if the whole thing
is simply impossible, I would rather avoid that round of RTFS at the moment...

Comments?