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[70.30.145.63]) by smtp.gmail.com with ESMTPSA id i7-20020ac84887000000b003b6a17e1996sm3546425qtq.83.2023.01.25.07.54.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Jan 2023 07:54:46 -0800 (PST) Date: Wed, 25 Jan 2023 10:54:44 -0500 From: Peter Xu To: James Houghton Cc: Mike Kravetz , linux-mm@kvack.org, Naoya Horiguchi , David Rientjes , Michal Hocko , Matthew Wilcox , David Hildenbrand , Muchun Song Subject: Re: A mapcount riddle Message-ID: References: MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Stat-Signature: axci3g8fbbhmuyxser3xkie6a9dt8mnb X-Rspam-User: X-Rspamd-Queue-Id: 7AA8680012 X-Rspamd-Server: rspam06 X-HE-Tag: 1674662089-686574 X-HE-Meta: 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 4rJQqM3a N2AbKHQLOteQl722Zn5A+5ph1C9nb9c8vvkMl+rDPSajpAxj/gJmylMJkVOKBGEiHC2e6+w0SYakbURQzJizkrXWhs509QZRFpOzB6cXZFtsK+J4nv42jEKjeR9r9iGINP5Nxg4o5+YciZ6UglO435LI3+UzLWot7XuvymbXdzPANymRHgIYZ/gZcX5C5mhHvKcO+DdBKltrxUcOgbVNmk54aYFtOCqhfVmTB7/jN/OoX624lslFYZX2k6fVOa5YmhjtOybGUdf7LIYetN7EYjtBP63vrj3hV1qcyIzr5ULTZNzmAbWB0wvUYj8AG3MlK8j5UEYlE9N//QR+H/3SZqNwtHdfmJIar65k3jzBZs2TmfUAz9oIVDY37yKgMjNtF0Wu8 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 Wed, Jan 25, 2023 at 07:26:49AM -0800, James Houghton wrote: > > At first thought this seems bad. However, I believe this has been the > > behavior since hugetlb PMD sharing was introduced in 2006 and I am > > unaware of any reported issues. I did a audit of code looking at > > mapcount. In addition to the above issue with smaps, there appears > > to be an issue with 'migrate_pages' where shared pages could be migrated > > without appropriate privilege. > > > > /* With MPOL_MF_MOVE, we migrate only unshared hugepage. */ > > if (flags & (MPOL_MF_MOVE_ALL) || > > (flags & MPOL_MF_MOVE && page_mapcount(page) == 1)) { > > if (isolate_hugetlb(page, qp->pagelist) && > > (flags & MPOL_MF_STRICT)) > > /* > > * Failed to isolate page but allow migrating pages > > * which have been queued. > > */ > > ret = 1; > > } > > This isn't the exact same problem you're fixing Mike, but I want to > point out a related problem. > > This is the generic-mm-equivalent of the hugetlb code above: > > static int migrate_page_add(struct page *page, struct list_head > *pagelist, unsigned long flags) > { > struct page *head = compound_head(page); > /* > * Avoid migrating a page that is shared with others. > */ > if ((flags & MPOL_MF_MOVE_ALL) || page_mapcount(head) == 1) { > if (!isolate_lru_page(head)) { > list_add_tail(&head->lru, pagelist); > mod_node_page_state(page_pgdat(head), > NR_ISOLATED_ANON + page_is_file_lru(head), > thp_nr_pages(head)); > ... > } > > If you have a partially PTE-mapped THP, page_mapcount(head) will not > accurately determine if a page is mapped in multiple VMAs or not (it > only tells you how many times the head page is mapped). > > For example... > 1) You could have the THP PMD-mapped in one VMA, and then one tail > page of the THP can be mapped in another. page_mapcount(head) will be > 1. > 2) You could have two VMAs map two separate tail pages of the THP, in > which case page_mapcount(head) will be 0. > > I bring this up because we have the same problem with HugeTLB > high-granularity mapping. Maybe a better match here is total_mapcount() rather than page_mapcount() (despite the overheads on the sub-page loop)? -- Peter Xu