Re: v4.6 kernel BUG at mm/rmap.c:1101!

[Andrea Arcangeli <aarcange@redhat.com>]

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 <aarcange@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 <aarcange@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;
 	/*