From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: [patch 3/5] mm: remove VM_BUG_ON(PageSlab()) from page_mapcount() Date: Wed, 27 May 2020 22:20:47 -0700 Message-ID: <20200528052047.tDxIOLXzt%akpm@linux-foundation.org> References: <20200527222015.62ba8592af63dae12ab58ffe@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from mail.kernel.org ([198.145.29.99]:45380 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725764AbgE1FUs (ORCPT ); Thu, 28 May 2020 01:20:48 -0400 In-Reply-To: <20200527222015.62ba8592af63dae12ab58ffe@linux-foundation.org> Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: akpm@linux-foundation.org, hughd@google.com, khlebnikov@yandex-team.ru, kirill.shutemov@linux.intel.com, linux-mm@kvack.org, mm-commits@vger.kernel.org, rientjes@google.com, stable@vger.kernel.org, torvalds@linux-foundation.org, vbabka@suse.cz From: Konstantin Khlebnikov Subject: mm: remove VM_BUG_ON(PageSlab()) from page_mapcount() Replace superfluous VM_BUG_ON() with comment about correct usage. Technically reverts commit 1d148e218a0d0566b1c06f2f45f1436d53b049b2 ("mm: add VM_BUG_ON_PAGE() to page_mapcount()"), but context have changed. Function isolate_migratepages_block() runs some checks out of lru_lock when choose pages for migration. After checking PageLRU() it checks extra page references by comparing page_count() and page_mapcount(). Between these two checks page could be removed from lru, freed and taken by slab. As a result this race triggers VM_BUG_ON(PageSlab()) in page_mapcount(). Race window is tiny. For certain workload this happens around once a year. page:ffffea0105ca9380 count:1 mapcount:0 mapping:ffff88ff7712c180 index:0x0 compound_mapcount: 0 flags: 0x500000000008100(slab|head) raw: 0500000000008100 dead000000000100 dead000000000200 ffff88ff7712c180 raw: 0000000000000000 0000000080200020 00000001ffffffff 0000000000000000 page dumped because: VM_BUG_ON_PAGE(PageSlab(page)) ------------[ cut here ]------------ kernel BUG at ./include/linux/mm.h:628! invalid opcode: 0000 [#1] SMP NOPTI CPU: 77 PID: 504 Comm: kcompactd1 Tainted: G W 4.19.109-27 #1 Hardware name: Yandex T175-N41-Y3N/MY81-EX0-Y3N, BIOS R05 06/20/2019 RIP: 0010:isolate_migratepages_block+0x986/0x9b0 Code in isolate_migratepages_block() was added in commit 119d6d59dcc0 ("mm, compaction: avoid isolating pinned pages") before adding VM_BUG_ON into page_mapcount(). This race has been predicted in 2015 by Vlastimil Babka (see link below). [akpm@linux-foundation.org: comment tweaks, per Hugh] Link: http://lkml.kernel.org/r/159032779896.957378.7852761411265662220.stgit@buzz Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/ Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1) Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") Signed-off-by: Konstantin Khlebnikov Acked-by: Hugh Dickins Acked-by: Kirill A. Shutemov Acked-by: Vlastimil Babka Cc: David Rientjes Cc: Signed-off-by: Andrew Morton --- include/linux/mm.h | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) --- a/include/linux/mm.h~mm-remove-vm_bug_onpageslab-from-page_mapcount +++ a/include/linux/mm.h @@ -782,6 +782,11 @@ static inline void *kvcalloc(size_t n, s extern void kvfree(const void *addr); +/* + * Mapcount of compound page as a whole, does not include mapped sub-pages. + * + * Must be called only for compound pages or any their tail sub-pages. + */ static inline int compound_mapcount(struct page *page) { VM_BUG_ON_PAGE(!PageCompound(page), page); @@ -801,10 +806,16 @@ static inline void page_mapcount_reset(s int __page_mapcount(struct page *page); +/* + * Mapcount of 0-order page; when compound sub-page, includes + * compound_mapcount(). + * + * Result is undefined for pages which cannot be mapped into userspace. + * For example SLAB or special types of pages. See function page_has_type(). + * They use this place in struct page differently. + */ static inline int page_mapcount(struct page *page) { - VM_BUG_ON_PAGE(PageSlab(page), page);