Re: [PATCH] mm: move rmap of mTHP upon CoW reuse

[David Hildenbrand]

On 25.09.25 12:50, Dev Jain wrote:
> On 25/09/25 4:07 pm, David Hildenbrand wrote:
> > On 25.09.25 12:33, Dev Jain wrote:
> > > On 25/09/25 2:46 pm, David Hildenbrand wrote:
> > > > On 25.09.25 10:54, Dev Jain wrote:
> > > > > At wp-fault time, when we find that a folio is exclusively mapped, we
> > > > > move
> > > > > folio->mapping to the faulting VMA's anon_vma, so that rmap overhead
> > > > > reduces. This is currently done for small folios (base pages) and
> > > > > PMD-mapped THPs. Do this for mTHP too.
> > > >
> > > > I deliberately didn't add this back then because I was not able to
> > > > convince myself easily that it is ok in all corner cases. So this
> > > > needs some thought.
> > >
> > > Thanks for your detailed reply.
> > >
> > >
> > > > We know that the folio is exclusively mapped to a single MM and that
> > > > there are no unexpected references from others (GUP pins, whatsoever).
> > > >
> > > > But a large folio might be
> > > >
> > > > (a) mapped into multiple VMAs (e.g., partial mprotect()) in the same MM
> > >
> > > I think we have the same problem then for PMD-THPs? I see that
> > > vma_adjust_trans_huge() only does a PMD split and not folio split.
> >
> > Sure, we can end up in this reuse function here for any large anon
> > folio, including PMD ones after a PMD->PTE remapping.
>
> Ah alright, I was thinking that something may go wrong through
> folio_move_anon_rmap() in do_huge_pmd_wp_page, but

Right, there we have sine VMA and single PTL.

--
Cheers

David / dhildenb