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 aVO8yKvR 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000027, 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 Sun, Mar 23, 2025 at 01:49:07PM +0100, David Hildenbrand wrote: > > > > > > c) In -next, there is now be the option to use folio lock + > > > folio_maybe_mapped_shared() == false. But it doesn't tell you into how many > > > VMAs a large folio is mapped into. > > > > > > In the following case: > > > > > > [ folio ] > > > [ VMA#1 ] [ VMA#2 ] > > > > > > c) would not tell you if you are fine modifying the folio when moving VMA#2. > > > > Right, I feel like prior checks made should assert this is not the case, > > however? But mapcount check should be a last ditch assurance? > > Something nice might be hiding in c) that might be able to handle a single > folio being covered by multiple vmas. > > I was thinking about the following: > > [ folio0 ] > [ VMA#0 ] > > Then we do a partial (old-school) mremap() > > [ folio0 ] [ folio0 ] > [ VMA#1 ] [ VMA#2 ] > > To then extend VMA#1 and fault in pages > > [ folio0 ][ folio1 ] [ folio0 ] > [ VMA#1 ] [ VMA#2 ] > > If that is possible (did not try!, maybe something prevents us from > extending VMA#1) mremap(MREMAP_RELOCATE_ANON) of VMA#1 / VMA#2 cannot work. > > We'd have to detect that scenario (partial mremap). You might be doing that > with the anon-vma magic, something different might be: Assume we flag large > folios if they were partially mremapped in any process. Do we have spare folio flags? :)) I always lose track of the situation with this and Matthew's levels of tolerance for it :P > > Then (with folio lock only) > > 1) folio_maybe_mapped_shared() == false: mapped into single process > 2) folio_maybe_partially_mremaped() == false: not scattered in virtual > address space > > It would be sufficient to check if the folio fully falls into the memap() > range to decide if we can adjust the folio index etc. > > We *might* be able to use that in the COW-reuse path for large folios to > perform a folio_move_anon_rmap(), which we currently only perform for small > folios / PMD-mapped folios (single mapping). Not sure yet if actually > multiple VMAs are involved. Interesting... this is the wp_can_reuse_anon_folio() stuff? I'll have a look into that! I'm concerned about partial cases moreso though, e.g.: mremap this <-----------> [ folio0 ] [ VMA#0 ] I mean, I'm leaning more towards just breaking up the folio, especialy if we consider a case like a biiig range: mremap this <---------------------------------------------------> [ folio0 ][ folio1 ][ folio2 ][ folio3 ][ folio4 ][ folio5 ] (say order-9 each) [ VMA#0 ] Then at this point, refusing to do the whole thing seems maybe a bad idea, at which point splitting the folios for folio0, 5 might be sensible. I guess a user is saying 'please, I really care about merging' so might well be willing to tolerate losing some of the huge page benefits, at least at the edges here. > > > > Just throwing it out there ... > > > > (actually at least one of the 'prior checks' for large folios are added in a > > later commit but still :P) > > > Yeah, I'm looking at the bigger picture; small folios are easy :P Yeah, back when life was simpler... :P > > > -- > Cheers, > > David / dhildenb > Cheers, Lorenzo