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 7qtGMwZ7 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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: List-Subscribe: List-Unsubscribe: On Mon, Mar 31, 2025 at 03:50:59PM +0100, Lorenzo Stoakes wrote: > 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 Am looking at this series again :) This function is very handy thanks, will use in the upcoming RFCv2! > > 2) folio_maybe_partially_mremaped() == false: not scattered in virtual > > address space This is intriguing, I think perhaps best to defer this to a later date :) have added a personal todo accordingly. We are able to fairly reasonably detect this case right now so it's not urgent. > > > > 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