Re: kernel oops on mmotm-2015-10-15-15-20

[Kirill A. Shutemov]

On Thu, Oct 29, 2015 at 04:58:29PM +0900, Minchan Kim wrote:
> On Thu, Oct 29, 2015 at 02:25:24AM +0200, Kirill A. Shutemov wrote:
> > On Thu, Oct 22, 2015 at 06:00:51PM +0900, Minchan Kim wrote:
> > > On Thu, Oct 22, 2015 at 10:21:36AM +0900, Minchan Kim wrote:
> > > > Hello Hugh,
> > > > 
> > > > On Wed, Oct 21, 2015 at 05:59:59PM -0700, Hugh Dickins wrote:
> > > > > On Thu, 22 Oct 2015, Minchan Kim wrote:
> > > > > > 
> > > > > > I added the code to check it and queued it again but I had another oops
> > > > > > in this time but symptom is related to anon_vma, too.
> > > > > > (kernel is based on recent mmotm + unconditional mkdirty for bug fix)
> > > > > > It seems page_get_anon_vma returns NULL since the page was not page_mapped
> > > > > > at that time but second check of page_mapped right before try_to_unmap seems
> > > > > > to be true.
> > > > > > 
> > > > > > Adding 4191228k swap on /dev/vda5.  Priority:-1 extents:1 across:4191228k FS
> > > > > > Adding 4191228k swap on /dev/vda5.  Priority:-1 extents:1 across:4191228k FS
> > > > > > page:ffffea0001cfbfc0 count:3 mapcount:1 mapping:ffff88007f1b5f51 index:0x600000aff
> > > > > > flags: 0x4000000000048019(locked|uptodate|dirty|swapcache|swapbacked)
> > > > > > page dumped because: VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma)
> > > > > 
> > > > > That's interesting, that's one I added in my page migration series.
> > > > > Let me think on it, but it could well relate to the one you got before.
> > > > 
> > > > I will roll back to mm/madv_free-v4.3-rc5-mmotm-2015-10-15-15-20
> > > > instead of next-20151021 to remove noise from your migration cleanup
> > > > series and will test it again.
> > > > If it is fixed, I will test again with your migration patchset, then.
> > > 
> > > I tested mmotm-2015-10-15-15-20 with test program I attach for a long time.
> > > Therefore, there is no patchset from Hugh's migration patch in there.
> > > And I added below debug code with request from Kirill to all test kernels.
> > 
> > It took too long time (and a lot of printk()), but I think I track it down
> > finally.
> >  
> > The patch below seems fixes issue for me. It's not yet properly tested, but
> > looks like it works.
> > 
> > The problem was my wrong assumption on how migration works: I thought that
> > kernel would wait migration to finish on before deconstruction mapping.
> > 
> > But turn out that's not true.
> > 
> > As result if zap_pte_range() races with split_huge_page(), we can end up
> > with page which is not mapped anymore but has _count and _mapcount
> > elevated. The page is on LRU too. So it's still reachable by vmscan and by
> > pfn scanners (Sasha showed few similar traces from compaction too).
> > It's likely that page->mapping in this case would point to freed anon_vma.
> > 
> > BOOM!
> > 
> > The patch modify freeze/unfreeze_page() code to match normal migration
> > entries logic: on setup we remove page from rmap and drop pin, on removing
> > we get pin back and put page on rmap. This way even if migration entry
> > will be removed under us we don't corrupt page's state.
> > 
> > Please, test.
> > 
> 
> kernel: On mmotm-2015-10-15-15-20 + pte_mkdirty patch + your new patch, I tested
> one I sent to you(ie, oops.c + memcg_test.sh)
> 
> page:ffffea00016a0000 count:3 mapcount:0 mapping:ffff88007f49d001 index:0x600001800 compound_mapcount: 0
> flags: 0x4000000000044009(locked|uptodate|head|swapbacked)
> page dumped because: VM_BUG_ON_PAGE(!page_mapcount(page))
> page->mem_cgroup:ffff88007f613c00

Ignore my previous answer. Still sleeping.

The right way to fix I think is something like:

diff --git a/mm/rmap.c b/mm/rmap.c
index 35643176bc15..f2d46792a554 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1173,20 +1173,12 @@ void do_page_add_anon_rmap(struct page *page,
 	bool compound = flags & RMAP_COMPOUND;
 	bool first;
 
-	if (PageTransCompound(page)) {
+	if (PageTransCompound(page) && compound) {
+		atomic_t *mapcount;
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
-		if (compound) {
-			atomic_t *mapcount;
-
-			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
-			mapcount = compound_mapcount_ptr(page);
-			first = atomic_inc_and_test(mapcount);
-		} else {
-			/* Anon THP always mapped first with PMD */
-			first = 0;
-			VM_BUG_ON_PAGE(!page_mapcount(page), page);
-			atomic_inc(&page->_mapcount);
-		}
+		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
+		mapcount = compound_mapcount_ptr(page);
+		first = atomic_inc_and_test(mapcount);
 	} else {
 		VM_BUG_ON_PAGE(compound, page);
 		first = atomic_inc_and_test(&page->_mapcount);
-- 
 Kirill A. Shutemov
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