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Sat, 14 Nov 2026 04:46:06 -0800 (PST) Message-ID: <5ff13e10-d8a6-43ed-a718-8b34ee08d4e4@gmail.com> Date: Sat, 14 Nov 2026 13:46:05 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/2] mm/huge_memory: find partially mapped shared folios with NO_PAGE_MAPCOUNT To: "David Hildenbrand (Arm)" , Zi Yan , Andrew Morton , Lorenzo Stoakes Cc: Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Rik van Riel , Vlastimil Babka , Harry Yoo , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Joanne Koong References: <20261111113535.330078-1-azpijr@gmail.com> <4411389c-2246-4af6-9b34-10d3f259aeb4@kernel.org> <3744f956-8179-45f3-9d87-c99c82b2f8db@kernel.org> Content-Language: en-US From: "Jose A. Perez de Azpillaga" In-Reply-To: <3744f956-8179-45f3-9d87-c99c82b2f8db@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D445B40005 X-Spam-Status: No, score=-4.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Stat-Signature: aefsgaf1tx4mt6ag1bkxqhz3ykkh8jc9 X-HE-Tag: 1794660368-362855 X-HE-Meta: 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 4kHBRpJp DDj3srld/rLL+k5spqEGJmfArtwn6DzMHL6VIf62+G3Fgr0qrVevxX8jyvXF9qqO6qQjHiM/jeRvL7E+/AWRDwAmea0oKyLbGBNZrGkMmPC5hmL13gIup6JwEI3axGr5BjKur52oPCD6HePGuMeQuqUIB4pYpnKergCWuVKYBTO8X/e8aYXbaYDeWkZQ1l9JL61enDQyBjPoM4uuc0jzrbg9N8ez2ctKcRt0nL66W9y1TvX2A2B3SoZQu72tDFBAetgdKjqylZbi+43wDRrXRMfPOygTSHezsdH97qPCllZsZTdzYIdm/kETvO4+awVI6122znVkKNjCKmIiRX4cv7zRHl2LQgETi7XbSGGe1suZ/LUfZESx5LFqGhkt4gL4oHOrPs1blIy7ttuHVr0dvz8U8IqVDW1aivIdmNS28FqJf7Bc692fRl25SgLGpE1HRVUGEEIzW30IbwqKJRTB+b3iGgLVI6FROnCg/fU2JuUrHwVQ= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/13/26 9:16 PM, David Hildenbrand (Arm) wrote: >>> (b) What is the overhead from having way too many folios in the deferred split list. >>> >> >> I don't know yet. I only measured the walk in a TCG guest, nothing on >> how long the list gets > > That would be really interesting. I assume there are some cases where a simple > fork+exec would already add pages to the deferred split list even thought they > are briefly after fully mapped by the parent. > I tested and, yes, it does. bash doing 200 fork+exec with 64K always queued 2 folios in total. [snip] >>> Maybe one could track it a bit broader? "any page in this set of 16 pages is >>> present". Reclaiming a single 4k page from a 2M folios is barely worth it. >>> >> >> yes, that works. maybe with the group size scaled to the folio, so the >> map is one long on the stack and 64K folio still gets one bit per page? >> > > As an alternative, could we also just look for the next page incrementally? > > Like, start with offset 0. When we see that, move to offset 1, when we see that, > move to offset 2 ... and once we saw all pages that way, stop the walk. > > I'd assume that, as long as VMAs don't get remapped, that should likely work in > most cases, right? > I tried it. if the parent keeps the second half and the child the first, a fully mapped folio looks partially mapped. maybe the one long map is better here? >> however, maybe if I don't have real evidence, better drop it? > > I'd be very interested in an efficient way to test if a folio is partially > mapped, and where the holes are. > > Knowing about the holes could guide a targeted buddy-like folio split. > > There was also the idea of letting folio_referenced() for that purpose, when we > area already doing an rmap walk. > okay, I could try that. would you rather have it without the shrinker walk? with no swap, reclaim wouldn't check those folios. -- cheers, jose a. p-a