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Shutemov" To: Konstantin Khlebnikov Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Hugh Dickins , Vlastimil Babka , David Rientjes , "Kirill A. Shutemov" Subject: Re: [PATCH v2] mm: remove VM_BUG_ON(PageSlab()) from page_mapcount() Message-ID: <20200525044439.f433y6enrpehdmid@box> References: <159032779896.957378.7852761411265662220.stgit@buzz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <159032779896.957378.7852761411265662220.stgit@buzz> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sun, May 24, 2020 at 04:43:18PM +0300, Konstantin Khlebnikov wrote: > 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). > > Signed-off-by: Konstantin Khlebnikov > Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") > Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/ > Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1) Acked-by: Kirill A. Shutemov -- Kirill A. Shutemov