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Date: Fri, 13 Jan 2017 20:11:21 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Alan J. Wylie" <alan@wylie.me.uk>,
        Thorsten Leemhuis <regressions@leemhuis.info>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn
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On Fri, Jan 13, 2017 at 08:08:27PM +0000, Al Viro wrote:

> Because it's "truncate to size", not "throw everything up to that point
> out".
> 
> We have some amount of data pushed into pipe (in this case - 0) and we
> have some buffers allocated by ..._get_pages() past the end of it.
> Some of that we want to keep (again, in this case - none) and have the next
> copy_to_iter() go after those, the rest we discard.

PS: 'size' argument of iov_iter_advance() is the second "some" in the
above - we tell it how much we want to advance by and everything past
that point is, in case of PIPE_ITER, discarded.