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Date: Fri, 29 Apr 2016 18:34:44 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [BUG] vfio device assignment regression with THP ref counting
 redesign
Message-ID: <20160429163444.GM11700@redhat.com>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Alex Williamson <alex.williamson@redhat.com>, kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org, "linux-mm@kvack.org" <linux-mm@kvack.org>

On Fri, Apr 29, 2016 at 10:06:11AM +0300, Kirill A. Shutemov wrote:
> Hm. I just woke up and haven't got any coffee yet, but I don't why my
> approach would be worse for performance. Both have the same algorithmic
> complexity.

Even before looking at the overall performance, I'm not sure your
patch is really fixing it all: you didn't touch reuse_swap_page which
is used by do_wp_page to know if it can call do_wp_page_reuse. Your
patch would still trigger a COW instead of calling do_wp_page_reuse,
but it would only happen if the page was pinned after the pmd split,
which is probably not what the testcase is triggering. My patch
instead fixed that too.

total_mapcount returns the wrong value for reuse_swap_page, which is
probably why you didn't try to use it there.

The main issue of my patch is that it has a performance downside that
is page_mapcount becomes expensive for all other usages, which is
better than breaking vfio but I couldn't use total_mapcount again
because it counts things wrong in reuse_swap_page.

Like I said there's room for optimizations so today I tried to
optimize more stuff...