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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=e1816BxD; spf=none (imf24.hostedemail.com: domain of ziy@nvidia.com has no SPF policy when checking 40.107.236.55) smtp.mailfrom=ziy@nvidia.com; dmarc=pass (policy=quarantine) header.from=nvidia.com X-Rspamd-Server: rspam06 X-HE-Tag: 1632443148-177379 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: --=_MailMate_BB7A2B28-15AB-4DDB-AD71-3CA2B9217E65_= Content-Type: text/plain On 23 Sep 2021, at 19:48, Hugh Dickins wrote: > On Thu, 23 Sep 2021, Zi Yan wrote: >> On 23 Sep 2021, at 17:54, Yang Shi wrote: >>> On Thu, Sep 23, 2021 at 2:10 PM Hugh Dickins wrote: >>>> >>>> NR_FILE_MAPPED being used for /proc/meminfo's "Mapped:" and a couple >>>> of other such stats files, and for a reclaim heuristic in mm/vmscan.c. >>>> >>>> Allow ourselves more slack in NR_FILE_MAPPED accounting (either count >>>> each pte as if it mapped the whole THP, or don't count a THP's ptes >>>> at all - you opted for the latter in the "Mlocked:" accounting), >>>> and I suspect subpage _mapcount could be abandoned. >>> >>> AFAIK, partial THP unmap may need the _mapcount information of every >>> subpage otherwise the deferred split can't know what subpages could be >>> freed. > > I believe Yang Shi is right insofar as the decision on whether it's worth > queuing for deferred split is being done based on those subpage _mapcounts. > That is a use I had not considered, and I've given no thought to how > important or not it is. > >> >> Could we just scan page tables of a THP during deferred split process >> instead? Deferred split is a slow path already, so maybe it can afford >> the extra work. > > But unless I misunderstand, actually carrying out the deferred split > already unmaps, uses migration entries, and remaps the remaining ptes: > needing no help from subpage _mapcounts to do those, and free the rest. You are right. unmap_page() during THP split is scanning the page tables already. For deciding whether to queue a THP for deferred split, we probably can keep PageDoubleMap bit to indicate if any subpage is PTE mapped. But without subpage _mapcount, detecting extra pins to a THP before split might be not as easy as with it. This means every THP split will need to perform unmap_page(), then check the remaining page_count to see if THP split is possible. That would also introduce extra system-wide overheads from unmapping pages. Am I missing anything? -- Best Regards, Yan, Zi --=_MailMate_BB7A2B28-15AB-4DDB-AD71-3CA2B9217E65_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEEh7yFAW3gwjwQ4C9anbJR82th+ooFAmFNGwMPHHppeUBudmlk aWEuY29tAAoJEJ2yUfNrYfqKTUUP/i6Zs51g5xfPrQGWv50OUQRKTOTS7awsOY+m BKiIAsVutyVQFpj7uLLFSbCcrptfflrweVeHTPx2Dnvdm441S0ZQpqcZSPoQHxPa 3d7GL5fC8Tr3pm9rV2O+tNi0AQY3xxw64vlAjKgbbUj0NbI93aG0KqXkDIaKTQcy q9LEgXAMYL1V4T1stA+rjq1PnmabANJgsEFkmt6vM2K7nIrlYr+sUHOaG5nFavtv bsWgfBFndYsbgpAGMNYJoU43MAxbLRAVyelNjoWcwJYOFtlgu7Xyjo5IVrs1rhzB ICTid48sc7Gj4WgvYeNcMMiupfyELSsIP1JjP6nBwNrw/kIY2DNqEs6SS5ZUrz7y 5mUdnvGlO7gRM26Fn5Bv/2tCl9/IlFgZsJwd+v8X4p/vIAl8ABrj5gHcvd/BO3yK mvMguZchMihvL0H5lDJrbsQELDuWQyysNr0fe92bV6iZ2iqMKgQqGKcNjtv1sLtp KSqLTslNslzxU3fzLZvc1WzZrob99+DDZgQtLLMP4sUOmcCssBgDhTXAtlI4oAm1 i1kfEIj7Nz5A4/MMK+cY34/5HJPSsmOLs27v746QXNon/OoLj7vlfYNSohrea9NJ l6lOPwUPDS7pVw0UG2rCgxR+tGUFsHPMUzotDv6BBQfQK8ilH+TPAX6aCiRddQ7g IVHz9T7W =aXh0 -----END PGP SIGNATURE----- --=_MailMate_BB7A2B28-15AB-4DDB-AD71-3CA2B9217E65_=--