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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id 95sm4462886otf.72.2020.05.24.11.50.13 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Sun, 24 May 2020 11:50:14 -0700 (PDT) Date: Sun, 24 May 2020 11:49:58 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils 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() In-Reply-To: <159032779896.957378.7852761411265662220.stgit@buzz> Message-ID: References: <159032779896.957378.7852761411265662220.stgit@buzz> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 24 May 2020, 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). Ah, kudos to Vlastimil. > > Signed-off-by: Konstantin Khlebnikov Thanks, Acked-by: Hugh Dickins For the BUG deletion. I got a bit confused by the comments at first: slight rewording suggested below. I had also intended a comment in page_mapcount(), on being used in that racy exceptional way by isolate_migratepages_block(): but now that the BUG is gone, I think you're right not to add that - it's not for the function to document its various callers, nor what's not in it. That kind of info belongs to the commit message, as you have done. > 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) > --- > include/linux/mm.h | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 5a323422d783..95f777f482ac 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -782,6 +782,11 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags) > > extern void kvfree(const void *addr); > > +/* > + * Mapcount of compound page as a whole, not includes mapped sub-pages. s/not includes/does not include/ > + * > + * 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 +806,15 @@ static inline void page_mapcount_reset(struct page *page) > > int __page_mapcount(struct page *page); > > +/* > + * Mapcount of 0-order page, for sub-page includes compound_mapcount(). s/, for sub-page/; when compound sub-page,/ > + * > + * 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); > - > if (unlikely(PageCompound(page))) > return __page_mapcount(page); > return atomic_read(&page->_mapcount) + 1;