On Mon, May 23, 2016 at 05:06:38PM +0300, Mika Westerberg wrote: > Hi, > > After upgrading kernel of my desktop system from v4.6-rc7 to v4.6, I've > started seeing following: > > [176611.093747] page:ffffea0000360000 count:1 mapcount:0 > mapping:ffff880034d2e0a1 index:0x1f9b06600 compound_mapcount: 0 > [176611.093751] flags: > 0x3fff8000044079(locked|uptodate|dirty|lru|active|head|swapbacked) > [176611.093752] page dumped because: VM_BUG_ON_PAGE(page->index != > linear_page_index(vma, address)) > [176611.093753] page->mem_cgroup:ffff88049e81b800 > [176611.093765] ------------[ cut here ]------------
This is a splitted pmd tail that is triggering a COW, but it's still a compound page because the physical split didn't happen yet. So like Kirill correctly pointed out, in such case we've to do compound_head because the page->mapping that has to be refiled to the local anon_vma is in the head. It's just a false positive VM_BUG_ON, the code itself is correct. Production kernels should be built with CONFIG_DEBUG_VM=n so this is not going to affect them and there's no bug for the production builds. Can you test this to shut off the false positive? >From 4db87e3e44837a0b038e58eaa3fea29db84723ec Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli <aarca...@redhat.com> Date: Mon, 23 May 2016 17:03:57 +0200 Subject: [PATCH 1/1] mm: thp: avoid false positive VM_BUG_ON_PAGE in page_move_anon_rmap() If the page_move_anon_rmap() is refiling a pmd-splitted THP mapped in a tail page from a pte, the "address" must be THP aligned in order for the page->index bugcheck to pass in the CONFIG_DEBUG_VM=y builds. Signed-off-by: Andrea Arcangeli <aarca...@redhat.com> --- mm/rmap.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/mm/rmap.c b/mm/rmap.c index 8a83993..e2e47ba9 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1098,7 +1098,10 @@ void page_move_anon_rmap(struct page *page, VM_BUG_ON_PAGE(!PageLocked(page), page); VM_BUG_ON_VMA(!anon_vma, vma); - VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page); + VM_BUG_ON_PAGE(page->index != + linear_page_index(vma, PageTransHuge(page) ? + address & HPAGE_PMD_MASK : + address), page); anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; /*