Re: [PATCH v2 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio_list()

[Lance Yang <ioworker0@gmail.com>]

On Thu, Apr 25, 2024 at 5:01 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 25.04.24 10:50, Lance Yang wrote:
> > Hey Zi, David,
>
> Hi,
>
> >
> > How about this change(diff against mm-unstable) as follows?
>
> goes into the right direction, please resent the whole thing, that will
> make it easier to review.

Got it. I‘ll keep that in mind, thanks!

>
> >
> > I'd like to add __try_to_unmap_huge_pmd() as a new internal function
> > specifically for unmapping PMD-mapped folios. If, for any reason, we cannot
> > unmap the folio, then we'll still split it as previously done.
> >
> > Currently, __try_to_unmap_huge_pmd() only handles lazyfree THPs, but it
> > can be extended to support other large folios that are PMD-mapped in the
> > future if needed.
> >
> > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > index 670218f762c8..0f906dc6d280 100644
> > --- a/include/linux/rmap.h
> > +++ b/include/linux/rmap.h
> > @@ -100,8 +100,6 @@ enum ttu_flags {
> >                                        * do a final flush if necessary */
> >       TTU_RMAP_LOCKED         = 0x80, /* do not grab rmap lock:
> >                                        * caller holds it */
> > -     TTU_LAZYFREE_THP        = 0x100, /* avoid splitting PMD-mapped THPs
> > -                                       * that are marked as lazyfree. */
> >   };
> >
> >   #ifdef CONFIG_MMU
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index a7913a454028..879c8923abfc 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1606,6 +1606,19 @@ void folio_remove_rmap_pmd(struct folio *folio, struct page *page,
> >   #endif
> >   }
> >
> > +static bool __try_to_unmap_huge_pmd(struct vm_area_struct *vma,
> > +                                 unsigned long addr, struct folio *folio)
> > +{
> > +     VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +     if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> > +             return discard_trans_pmd(vma, addr, folio);
> > +#endif
> > +
> > +     return false;
> > +}
> > +
> >   /*
> >    * @arg: enum ttu_flags will be passed to this argument
> >    */
> > @@ -1631,14 +1644,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >       if (flags & TTU_SYNC)
> >               pvmw.flags = PVMW_SYNC;
> >
> > -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > -     if (flags & TTU_LAZYFREE_THP)
> > -             if (discard_trans_pmd(vma, address, folio))
> > +     if (flags & TTU_SPLIT_HUGE_PMD) {
> > +             if (__try_to_unmap_huge_pmd(vma, address, folio))
> >                       return true;
> > -#endif
> > -
> > -     if (flags & TTU_SPLIT_HUGE_PMD)
> >               split_huge_pmd_address(vma, address, false, folio);
> > +     }
> >
>
> I was wondering if we can better integrate that into the pagewalk below.
>
> That is, don't do the TTU_SPLIT_HUGE_PMD immediately. Start the pagewalk
> first. If we walk a PMD, try to unmap it. Only if that fails, split it.

Nice. Thanks for the suggestion!
I'll work on integrating it into the pagewalk as you suggested.

>
> Less working on "vma + address" and instead directly on PMDs.

Yes, some of the work on "vma + address" can be avoided :)

Thanks again for the review!
Lance

>
> --
> Cheers,
>
> David / dhildenb
>