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 YGZVzjcJ RseOCjQ7LQ7NQIsHpswPRFZGW4suu6P8wPDpKBAVDcyOf/Eu5bzDQqosqXEL2+QZTNhdxMByjlJAsCQkmPDWafKVLEcpSfjHhpg4g6WKpP2f1owK6vqOdfOIQJseJEDUKAOnLVzGUDTcEGPrPKDwtAub1Wp926I7IKZ81w4MBw7iyKkBn80Yr/qBm+E6Qaof64OUkDCA/tpEuk/igDCVAl3eNFYIZSj6mEoQzSegKvYX05zms++xz7t6bS5LhsJdhYevCUjKiLpGSuk8iBgngmqsIBh/EaY7WgbsHqYGm1cz5nLp4Hhqz4qeKbwxUw2/4nfDxZp5cAMX3aZTcZQ+e7BebuS7JYa6HkNSkDp84fBq3ECVJ4vEtL2ioPimNa3muvUUX 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 01/25/23 09:24, Michal Hocko wrote: > On Tue 24-01-23 12:56:24, Mike Kravetz 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; > > } > > Could you elaborate what is problematic about that? The whole pmd > sharing is a cooperative thing. So if some of the processes decides to > migrate the page then why that should be a problem for others sharing > that page via page table? Am I missing something obvious? Nothing obvious. It is just that the semantics seem to be that you can only move shared pages if you have CAP_SYS_NICE. Certainly cooperation is implied for shared PMDs, but I would guess that most applications are not even aware they are sharing PMDs. Consider a group of processes sharing a hugetlb mapping. If the mapping is PUD_SIZE - huge_page_size, there is no sharing of PMDs and a process without CAP_SYS_NICE can not migrate the shared pages. However, if nothing else changes and the mapping size is PUD_SIZE (and appropriately aligned) the PMDs are shared. Should we allow a process to migrate shared pages without CAP_SYS_NICE in this case? -- Mike Kravetz