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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>