Re: [RFC PATCH 1/7] mm/mremap: introduce more mergeable mremap via MREMAP_RELOCATE_ANON

[Lorenzo Stoakes]

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