Re: [RFC PATCH 0/9] introduce PGTY_mgt_entry page_type

[David Hildenbrand]

On 24.07.25 11:12, David Hildenbrand wrote:
> On 24.07.25 11:09, Huan Yang wrote:
> > 在 2025/7/24 16:59, David Hildenbrand 写道:
> > > On 24.07.25 10:44, Huan Yang wrote:
> > > > Summary
> > > > ==
> > > > This patchset reuses page_type to store migrate entry count during the
> > > > period from migrate entry setup to removal, enabling accelerated VMA
> > > > traversal when removing migrate entries, following a similar
> > > > principle to
> > > > early termination when folio is unmapped in try_to_migrate.
> > >
> > > I absolutely detest (ab)using page types for that, so no from my side
> > > unless I am missing something important.
> > >
> > > > In my self-constructed test scenario, the migration time can be reduced
> > >
> > > How relevant is that in practice?
> >
> > IMO, any folio mapped < nr vma in mapping(anon_vma, addresss_space),
> > will benefit from this.
> >
> > So, all pages that have been COW-ed by child processes can be skipped.
>
> For small anon folios, you could use the anon-exclusive marker to derive
> "there can only be a single mapping".
>
> It's stored alongside the migration entry.
>
> So once you restored that single migration entry, you can just stop the
> walk.

Essentially, something (untested) like this:

diff --git a/mm/migrate.c b/mm/migrate.c
index 425401b2d4e14..aa5bf96b1daee 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -421,6 +421,15 @@ static bool remove_migration_pte(struct folio *folio,
 
                /* No need to invalidate - it was non-present before */
                update_mmu_cache(vma, pvmw.address, pvmw.pte);
+
+               /*
+                * If the small anon folio is exclusive, here can be exactly one
+                * page mapping -- the one we just restored.
+                */
+               if (!folio_test_large(folio) && (rmap_flags & RMAP_EXCLUSIVE)) {
+                       page_vma_mapped_walk_done(&pvmw);
+                       break;
+               }
        }
 
        return true;


--
Cheers,

David / dhildenb