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

[David Hildenbrand <david@redhat.com>]

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