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Morton , Jonathan Corbet , Mike Kravetz , Hugh Dickins , Ryan Roberts , Yin Fengwei , Yang Shi Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios Date: Fri, 11 Aug 2023 11:14:39 -0400 X-Mailer: MailMate (1.14r5964) Message-ID: <9D4A9EE8-8CEF-4B08-9A32-149B6C548AB8@nvidia.com> In-Reply-To: References: <20230809083256.699513-1-david@redhat.com> <7e31254d-8889-7e79-50e1-2630bd493d59@redhat.com> <73d6d29f-9947-9b50-3b94-77f1ee547387@redhat.com> Content-Type: multipart/signed; boundary="=_MailMate_6E3800E0-F9D0-4996-BFD7-F4A73F9A56DF_="; micalg=pgp-sha512; protocol="application/pgp-signature" X-ClientProxiedBy: BL1PR13CA0061.namprd13.prod.outlook.com (2603:10b6:208:2b8::6) To DS7PR12MB5744.namprd12.prod.outlook.com (2603:10b6:8:73::18) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB5744:EE_|MW4PR12MB7238:EE_ X-MS-Office365-Filtering-Correlation-Id: 0f072c1a-ed3c-4b52-e165-08db9a7dafe8 X-MS-Exchange-SenderADCheck: 1 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 EU7MT9yQ 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 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_6E3800E0-F9D0-4996-BFD7-F4A73F9A56DF_= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable On 11 Aug 2023, at 11:03, Peter Xu wrote: > On Thu, Aug 10, 2023 at 11:59:25PM +0200, David Hildenbrand wrote: >> On 10.08.23 23:54, Matthew Wilcox wrote: >>> On Thu, Aug 10, 2023 at 05:48:19PM -0400, Peter Xu wrote: >>>>> Yes, that comment from Hugh primarily discusses how we could possib= ly >>>>> optimize the loop, and if relying on folio_nr_pages_mapped() to red= uce the >>>>> iterations would be racy. As far as I can see, there are cases wher= e "it >>>>> would be certainly a bad idea" :) >>>> >>>> Is the race described about mapcount being changed right after it's = read? >>>> Are you aware of anything specific that will be broken, and will be = fixed >>>> with this patch? >>> >>> The problem is that people check the mapcount while holding no locks;= >>> not the PTL, not the page lock. So it's an unfixable race. >>> >>>> Having a total mapcount does sound helpful if partial folio is commo= n >>>> indeed. >>>> >>>> I'm curious whether that'll be so common after the large anon folio = work - >>>> isn't it be sad if partial folio will be a norm? It sounds to me th= at's >>>> the case when small page sizes should be used.. and it's prone to wa= ste? >>> >>> The problem is that entire_mapcount isn't really entire_mapcount. >>> It's pmd_mapcount. I have had thoughts about using it as entire_mapc= ount, >>> but it gets gnarly when people do partial unmaps. So the _usual_ cas= e >>> ends up touching every struct page. Which sucks. Also it's one of t= he >>> things which stands in the way of shrinking struct page. >> >> Right, so one current idea is to have a single total_mapcount and look= into >> removing the subpage mapcounts (which will require first removing >> _nr_pages_mapped, because that's still one of the important users). >> >> Until we get there, also rmap code has to do eventually "more tracking= " and >> might, unfortunately, end up slower. >> >>> >>> But it's kind of annoying to explain all of this to you individually.= >>> There have been hundreds of emails about it over the last months on >>> this mailing list. It would be nice if you could catch up instead of= >>> jumping in. >> >> To be fair, a lot of the details are not readily available and in the = heads >> of selected people :) >> >> Peter, if you're interested, we can discuss the current plans, issues = and >> ideas offline! > > Thanks for offering help, David. > > Personally I still am unclear yet on why entire_mapcount cannot be used= as > full-folio mapcounts, and why "partial unmap" can happen a lot (I don't= > expect), but yeah I can try to catch up to educate myself first. Separate entire_mapcount and per-page mapcount are needed to maintain pre= cise NR_{ANON,FILE}_MAPPED and NR_ANON_THPS. I wrote some explanation (third p= aragraph) at: https://lore.kernel.org/linux-mm/A28053D6-E158-4726-8BE1-B9F4960AD570= @nvidia.com/. Let me know if it helps. > > The only issue regarding an offline sync-up is that even if David will = help > Peter on catching up the bits, it'll not scale when another Peter2 had = the > same question.. So David, rather than I waste your time on helping one= > person, let me try to catch up with the public threads - I'm not sure h= ow > far I can go myself; otoh thread links will definitely be helpful to be= > replied here, so anyone else can reference too. I collected a list (wh= ich > can be enriched) of few threads that might be related, just in case hel= pful > to anyone besides myself: > > [PATCH 0/2] don't use mapcount() to check large folio sharing > https://lore.kernel.org/r/20230728161356.1784568-1-fengwei.yin@intel.co= m > > [PATCH v1-v2 0/3] support large folio for mlock > https://lore.kernel.org/r/20230728070929.2487065-1-fengwei.yin@intel.co= m > https://lore.kernel.org/r/20230809061105.3369958-1-fengwei.yin@intel.co= m > > [PATCH v1 0/4] Optimize mmap_exit for large folios > https://lore.kernel.org/r/20230810103332.3062143-1-ryan.roberts@arm.com= > > [PATCH v4-v5 0/5] variable-order, large folios for anonymous memory > https://lore.kernel.org/linux-mm/20230726095146.2826796-1-ryan.roberts@= arm.com/ > https://lore.kernel.org/r/20230810142942.3169679-1-ryan.roberts@arm.com= > > [PATCH v3-v4 0/3] Optimize large folio interaction with deferred split > (I assumed Ryan's this one goes into the previous set v5 finally, so ju= st > the discussions as reference) > https://lore.kernel.org/r/20230720112955.643283-1-ryan.roberts@arm.com > https://lore.kernel.org/r/20230727141837.3386072-1-ryan.roberts@arm.com= > > [RFC PATCH v2 0/4] fix large folio for madvise_cold_or_pageout() > https://lore.kernel.org/r/20230721094043.2506691-1-fengwei.yin@intel.co= m > > I'm not sure how far I'll go; maybe I'll start working on something els= e > before I finish all of them. I'll see.. > > Not allowing people to jump in will definitely cause less interactions = and > less involvement/open-ness for the mm community, as sometimes people ca= n't > easily judge when it's proper to jump in. > > IMHO the ideal solution is always keep all discussions public (either > meetings with recordings, or shared online documents, always use on-lis= t > discussions, etc.), then share the links. > > -- = > Peter Xu -- Best Regards, Yan, Zi --=_MailMate_6E3800E0-F9D0-4996-BFD7-F4A73F9A56DF_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEE6rR4j8RuQ2XmaZol4n+egRQHKFQFAmTWUF8PHHppeUBudmlk aWEuY29tAAoJEOJ/noEUByhUH+YP/0bDLQeE5iGqZmI/A5O0o72OlUekgbr84kpI iObR045d8OyKoWoJtQo+Y9szMQh+FXCMGW6tY+9HH/C+knXYMiIcfWTHO5N633K9 S539hSrcQSCgVH4c/wMo7ejgN0VIBIKFKkZ1BXmTMLryBagfjjIMTnyeXkTHgC6C RyKvXJEfv9SmSAMgJDFFfbdSt1yILSBjpH8+RIDifpZRUEYvmZDiuyuDkYQ16t7z +wHwSiXVNNyEGEf3CdxTFRkgZKrQ7YOx1iIOTE1DQkoaSqQip/FKKtXp3GGJs8Yl sWCVxKUYCuFj1kfKl4+xaRrS4liG2wnVhrIemVck2P/mphBSS92FVrk8jo8qmPc6 R1ogcYx4mi9s7pCtwytnWYdLn+E4eWy2/99EPjR9UwxgLZLycH0MOh++gL5xz5zj 66lt6Qw7MmUBsTMY4FmnB6pc84eP5fYKdARaoQs2j/kgThPtWOz/Q1xxql16joqM mH/Ch4upKgdG2IYzHh+uXderYW9UHikFzrMJMrbj1qgq4tEJnAFz7SxH3v17rn+z MVFElcflbiIWsZjpY2EJvExX8zCF/dHsIK6a8GiIyXdGgAQBkG4tsCmhdBHFVctH yTHLEZbX4bKmrgTMK6DTNAKVBnPH70LYu6zm+vn3HPvl5Cagj6rXPXU5C7dvdmxw DTncHBLj =skLA -----END PGP SIGNATURE----- --=_MailMate_6E3800E0-F9D0-4996-BFD7-F4A73F9A56DF_=--