Re: [BUG] vfio device assignment regression with THP ref counting redesign

[Kirill A. Shutemov]

On Fri, Apr 29, 2016 at 01:21:27AM +0200, Andrea Arcangeli wrote:
> Hello Alex and Kirill,
> 
> On Thu, Apr 28, 2016 at 12:58:08PM -0600, Alex Williamson wrote:
> > > > specific fix to this code is not applicable.  It also still occurs on
> > > > kernels as recent as v4.6-rc5, so the issue hasn't been silently fixed
> > > > yet.  I'm able to reproduce this fairly quickly with the above test,
> > > > but it's not hard to imagine a test w/o any iommu dependencies which
> > > > simply does a user directed get_user_pages_fast() on a set of userspace
> > > > addresses, retains the reference, and at some point later rechecks that
> > > > a new get_user_pages_fast() results in the same page address.  It
> 
> Can you try to "git revert 1f25fe20a76af0d960172fb104d4b13697cafa84"
> and then apply the below patch on top of the revert?
> 
> Totally untested... if I missed something and it isn't correct, I hope
> this brings us in the right direction faster at least.
> 
> Overall the problem I think is that we need to restore full accuracy
> and we can't deal with false positive COWs (which aren't entirely
> cheap either... reading 512 cachelines should be much faster than
> copying 2MB and using 4MB of CPU cache). 32k vs 4MB. The problem of
> course is when we really need a COW, we'll waste an additional 32k,
> but then it doesn't matter that much as we'd be forced to load 4MB of
> cache anyway in such case. There's room for optimizations but even the
> simple below patch would be ok for now.
> 
> From 09e3d1ff10b49fb9c3ab77f0b96a862848e30067 Mon Sep 17 00:00:00 2001
> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Date: Fri, 29 Apr 2016 01:05:06 +0200
> Subject: [PATCH 1/1] mm: thp: calculate page_mapcount() correctly for THP
>  pages
> 
> This allows to revert commit 1f25fe20a76af0d960172fb104d4b13697cafa84
> and it provides fully accuracy with wrprotect faults so page pinning
> will stop causing false positive copy-on-writes.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
>  mm/util.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 6cc81e7..a0b9f63 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -383,9 +383,10 @@ struct address_space *page_mapping(struct page *page)
>  /* Slow path of page_mapcount() for compound pages */
>  int __page_mapcount(struct page *page)
>  {
> -	int ret;
> +	int ret = 0, i;
>  
> -	ret = atomic_read(&page->_mapcount) + 1;
> +	for (i = 0; i < HPAGE_PMD_NR; i++)
> +		ret = max(ret, atomic_read(&page->_mapcount) + 1);
>  	page = compound_head(page);
>  	ret += atomic_read(compound_mapcount_ptr(page)) + 1;
>  	if (PageDoubleMap(page))

You are right about the cause. I spend some time on wrong path: I was only
able to trigger the bug with numa balancing enabled, so I assumed
something is wrong in that code...

I would like to preserve current page_mapcount() behaviouts.
I think this fix is better:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 86f9f8b82f8e..163c10f48e1b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1298,15 +1298,9 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
        VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
        /*
         * We can only reuse the page if nobody else maps the huge page or it's
-        * part. We can do it by checking page_mapcount() on each sub-page, but
-        * it's expensive.
-        * The cheaper way is to check page_count() to be equal 1: every
-        * mapcount takes page reference reference, so this way we can
-        * guarantee, that the PMD is the only mapping.
-        * This can give false negative if somebody pinned the page, but that's
-        * fine.
+        * part.
         */
-       if (page_mapcount(page) == 1 && page_count(page) == 1) {
+       if (total_mapcount(page) == 1) {
                pmd_t entry;
                entry = pmd_mkyoung(orig_pmd);
                entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
-- 
 Kirill A. Shutemov