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

>
> 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.

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

--
Cheers,

David / dhildenb


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