On 11/30/2015 03:24 PM, Sasha Levin wrote: > Hi all, > > I've hit the following while fuzzing with trinity on the latest -next kernel: > > > [ 850.305385] page:ffffea001a5a0f00 count:0 mapcount:1 mapping:dead000000000400 index:0x1ffffffffff > [ 850.306773] flags: 0x2fffff80000000() > [ 850.307175] page dumped because: VM_BUG_ON_PAGE(1 && PageTail(page)) > [ 850.308027] page_owner info is not active (free page?) > [ 850.308925] ------------[ cut here ]------------ > [ 850.309614] kernel BUG at include/linux/page-flags.h:326! > [ 850.310333] invalid opcode: 0000 [#1] PREEMPT SMP KASAN > [ 850.311176] Modules linked in: > [ 850.311650] CPU: 5 PID: 7051 Comm: trinity-c129 Not tainted 4.4.0-rc2-next-20151127-sasha-00012-gf0498ca-dirty #2661 > [ 850.313115] task: ffff8806eaf08000 ti: ffff8806b1170000 task.ti: ffff8806b1170000 > [ 850.314085] RIP: __munlock_pagevec (include/linux/page-flags.h:326 mm/mlock.c:296) That's TestClearPageMlocked(page) which has PF_NO_TAIL. The page dump suggests the page was freed between the check triggering, and the page being dumped. But being on munlock's pagevec should pin the page. So a pin/unpin mismatch somewhere, together with a race? Moreover, a PageTail(page) shouldn't even get on the pagevec, munlock_vma_pages_range() skips tail pages. So another race that made the page a Tail after it was added to pagevec? Or maybe __munlock_pagevec_fill() encountered a tail page, and since it assumes that it can't happen, there's no check. Maybe a VM_BUG_ON_PAGE() there would catch this earlier? Could be related to "thp: allow mlocked THP again". Ah, __munlock_pagevec_fill() does a get_page(), which would increase page->count on the compound head, which could also explain the mismatch. ------8<------ diff --git a/mm/mlock.c b/mm/mlock.c index af421d8bd6da..156d2840aa62 100644 --- a/mm/mlock.c +++ b/mm/mlock.c @@ -393,7 +393,9 @@ static unsigned long __munlock_pagevec_fill(struct pagevec *pvec, if (!page || page_zone_id(page) != zoneid) break; + VM_BUG_ON_PAGE(PageTail(page), page); get_page(page); + /* * Increase the address that will be returned *before* the * eventual break due to pvec becoming full by adding the page -- 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>