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 ZQHrVwuP Bls+rDRfeF9rqbo9lF3oDo+cBzs5qR6cwJiXyVJ15Smr1HOnUhR+lYK4SLb7N/Rb/vNErbWotFjwFqb1u+GXyhoB4wEbrSEkiplGQHM83uL0N0wCEFmRj9oBtmqhWVgNuHHMd/R5839U5N8ixsvBJA6bXvKckXh4AkEGlOp4740lcYePcimoJ2yVfiJozCWExFOrFrRBwc3qBj1m6dr1GhyfcoWWek4S97ROjoaPnI8P4Qf/6klcmrvRQoaB59Oss+7PsE07qZiqBNCh6LMRilmG2DlYf98gri5JwFPiEsBJ//y7BZW4TBEAReFvPN92YTWpohph5zsMgJqa9N96Eo/Ve4p7LO9qd5BogBQkHbZxnkQiA1xbIlQwrFJMvmz6AcnoKWVMHsg26bp66LxlqEFriU2xrZkna6cYnm12z1o5mQerlITVy52wteLb5IgdgvRZlc8GIwmvasf6RRW0B6FT7KiTlHkEMeGou 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: List-Subscribe: List-Unsubscribe: On 17 Feb 2026, at 16:04, David Hildenbrand (Arm) wrote: > Hi, > > although I like mapcounts very much, I'd rather prefer to not have mapcou= nt work on my todo list. > > We now have CONFIG_NO_PAGE_MAPCOUNT in the kernel that doesn't touch any = mapcount values of tail pages, which is great. But we still have CONFIG_PAG= E_MAPCOUNT around, being used as default. > > > To make my dream come true, some things I have in mind are still pending.= In particular, I want to: > > (a) Support mapping of folios > PMD through PMDs. > > (b) Get rid of CONFIG_PAGE_MAPCOUNT to stop messing with > page->_mapcount on tail pages and to cleanup the rmap code. > > (c) Better detect partially-mapped anon folios with > CONFIG_NO_PAGE_MAPCOUNT. > > + some other small things. > > > I discussed some of these challenges at LSF/MM 2024 [1], before we had CO= NFIG_NO_PAGE_MAPCOUNT. No we have it and we can discuss the next steps. > > > Sorting out (a) is fairly easy once we removed CONFIG_PAGE_MAPCOUNT: we'l= l primarily have to split folio->_entire_mapcount into folio->_pmd_mapcount= and folio->_pud_mapcount. Then, for PMD sized folio, _pmd_mapcount is its =E2=80=9C_entire_mapcount= =E2=80=9D, for PUD sized folio, _pud_mapcount is its =E2=80=9C_entire_mapcount=E2=80=9D. F= or mulit-PMD or multi-PUD folio, _pmd_mapcount and _pud_mapcount are similar to _nr_pages_mapped but with PMD_NR/PUD_NR multiplier. Maybe we would have _pte_mapcount instead of _nr_pages_mapped? > > Sorting out (b) requires switching to CONFIG_NO_PAGE_MAPCOUNT first, whic= h will imply some imprecision with large folios to: > > (1) Process memory stats: Pss + Uss accounting like "Pss" and "Shared_" > vs "Private_" in /proc/$PID/smaps and /proc/$PID/smaps_rollup > > (2) PM_MMAP_EXCLUSIVE flag in /proc/$PID/pagemap > > (3) System memory stats: "mapped" memory like "AnonPages", "Mapped" > and "Shmem" in /proc/meminfo > > And some other smaller things. While I think that all changes here should= be fine, I want to be a bit careful and have a discussion on how to tackle= it without realizing in a couple of releases that some use cases still req= uire CONFIG_PAGE_MAPCOUNT. > > Sorting out (c) is a harder nut to crack, and I wonder to which degree we= care and whether I am being too careful. I have some ideas that I want to = discuss. One idea is to just remove the deferred split lists and let memory= reclaim deal with that: Or let a workqueue to walk rmap to check if an unmapped subpage indeed has no mapping left and put that folio in deferred_split list. Or let the deferred_list_scan does the rmap walk and decide whether to split the folio= . > but that one might be discussed in another session I'll propose around th= e deferred split lists. > > > [1] https://lwn.net/Articles/1013649/ > > --=20 > Cheers, > > David Best Regards, Yan, Zi