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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Oct 30, 2015 at 04:03:50PM +0900, Minchan Kim wrote:
> On Thu, Oct 29, 2015 at 11:52:06AM +0200, Kirill A. Shutemov wrote:
> > 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);
> > -- 
> 
> kernel: On mmotm-2015-10-15-15-20 + pte_mkdirty patch + freeze/unfreeze patch + above patch,
> 
> 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
> 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
> Adding 4191228k swap on /dev/vda5.  Priority:-1 extents:1 across:4191228k FS
> BUG: Bad rss-counter state mm:ffff880058d2e580 idx:1 val:512
> 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
> 
> <SNIP>
> 
> 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
> Adding 4191228k swap on /dev/vda5.  Priority:-1 extents:1 across:4191228k FS
> BUG: Bad rss-counter state mm:ffff880046980700 idx:1 val:511
> BUG: Bad rss-counter state mm:ffff880046980700 idx:2 val:1

Hm. I was not able to trigger this and don't see anything obviuous what can
lead to this kind of missmatch :-/

I found one more bug: clearing of PageTail can be visible to other CPUs
before updated page->flags on the page.

I don't think this bug is connected to what you've reported, but worth
testing.

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 5e0fe82a0fae..12bd8c5a4409 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2934,6 +2934,13 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 
 	smp_wmb(); /* make pte visible before pmd */
 	pmd_populate(mm, pmd, pgtable);
+
+	if (freeze) {
+		for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
+			page_remove_rmap(page + i, false);
+			put_page(page + i);
+		}
+	}
 }
 
 void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
@@ -3079,6 +3086,8 @@ static void freeze_page_vma(struct vm_area_struct *vma, struct page *page,
 		if (pte_soft_dirty(entry))
 			swp_pte = pte_swp_mksoft_dirty(swp_pte);
 		set_pte_at(vma->vm_mm, address, pte + i, swp_pte);
+		page_remove_rmap(page, false);
+		put_page(page);
 	}
 	pte_unmap_unlock(pte, ptl);
 }
@@ -3117,8 +3126,6 @@ static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page,
 		return;
 	pte = pte_offset_map_lock(vma->vm_mm, pmd, address, &ptl);
 	for (i = 0; i < HPAGE_PMD_NR; i++, address += PAGE_SIZE, page++) {
-		if (!page_mapped(page))
-			continue;
 		if (!is_swap_pte(pte[i]))
 			continue;
 
@@ -3128,6 +3135,9 @@ static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page,
 		if (migration_entry_to_page(swp_entry) != page)
 			continue;
 
+		get_page(page);
+		page_add_anon_rmap(page, vma, address, false);
+
 		entry = pte_mkold(mk_pte(page, vma->vm_page_prot));
 		entry = pte_mkdirty(entry);
 		if (is_write_migration_entry(swp_entry))
@@ -3181,8 +3191,6 @@ static int __split_huge_page_tail(struct page *head, int tail,
 	 */
 	atomic_add(mapcount + 1, &page_tail->_count);
 
-	/* after clearing PageTail the gup refcount can be released */
-	smp_mb__after_atomic();
 
 	page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
 	page_tail->flags |= (head->flags &
@@ -3195,6 +3203,12 @@ static int __split_huge_page_tail(struct page *head, int tail,
 			 (1L << PG_unevictable)));
 	page_tail->flags |= (1L << PG_dirty);
 
+	/*
+	 * After clearing PageTail the gup refcount can be released.
+	 * Page flags also must be visible before we make the page non-compound.
+	 */
+	smp_wmb();
+
 	clear_compound_head(page_tail);
 
 	if (page_is_young(head))
diff --git a/mm/rmap.c b/mm/rmap.c
index 35643176bc15..e4f8d9fb1c3d 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 (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);
@@ -1201,7 +1193,6 @@ void do_page_add_anon_rmap(struct page *page,
 		 * disabled.
 		 */
 		if (compound) {
-			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
 			__inc_zone_page_state(page,
 					      NR_ANON_TRANSPARENT_HUGEPAGES);
 		}
-- 
 Kirill A. Shutemov

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