Re: [RFC PATCH] mm: don't promote exclusive file folios of dying processes

[Zi Yan <ziy@nvidia.com>]

On 12 Apr 2025, at 4:58, Barry Song wrote:

> From: Barry Song <v-songbaohua@oppo.com>
>
> Promoting exclusive file folios of a dying process is unnecessary and
> harmful. For example, while Firefox is killed and LibreOffice is
> launched, activating Firefox's young file-backed folios makes it
> harder to reclaim memory that LibreOffice doesn't use at all.
>
> An exiting process is unlikely to be restarted right away—it's
> either terminated by the user or killed by the OOM handler.

The proposal looks reasonable to me. Do you have any performance number
about the improvement?

>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>  mm/huge_memory.c |  4 ++--
>  mm/internal.h    | 19 +++++++++++++++++++
>  mm/memory.c      |  9 ++++++++-
>  3 files changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e97a97586478..05b83d2fcbb6 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2264,8 +2264,8 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  			 * Use flush_needed to indicate whether the PMD entry
>  			 * is present, instead of checking pmd_present() again.
>  			 */
> -			if (flush_needed && pmd_young(orig_pmd) &&
> -			    likely(vma_has_recency(vma)))
> +			if (!exclusive_folio_of_dying_process(folio, vma) && flush_needed &&
> +			    pmd_young(orig_pmd) && likely(vma_has_recency(vma)))
>  				folio_mark_accessed(folio);
>  		}
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 4e0ea83aaf1c..666de96a293d 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -11,6 +11,7 @@
>  #include <linux/khugepaged.h>
>  #include <linux/mm.h>
>  #include <linux/mm_inline.h>
> +#include <linux/oom.h>
>  #include <linux/pagemap.h>
>  #include <linux/pagewalk.h>
>  #include <linux/rmap.h>
> @@ -130,6 +131,24 @@ static inline int folio_nr_pages_mapped(const struct folio *folio)
>  	return atomic_read(&folio->_nr_pages_mapped) & FOLIO_PAGES_MAPPED;
>  }
>
> +/*
> + * Return true if a folio is exclusive and belongs to an exiting or
> + * oom-reaped process; otherwise, return false.
> + */
> +static inline bool exclusive_folio_of_dying_process(struct folio *folio,
> +		struct vm_area_struct *vma)
> +{
> +	if (folio_maybe_mapped_shared(folio))
> +		return false;
> +
> +	if (!atomic_read(&vma->vm_mm->mm_users))
> +		return true;
> +	if (check_stable_address_space(vma->vm_mm))
> +		return true;
> +
> +	return false;
> +}
> +
>  /*
>   * Retrieve the first entry of a folio based on a provided entry within the
>   * folio. We cannot rely on folio->swap as there is no guarantee that it has
> diff --git a/mm/memory.c b/mm/memory.c
> index b9e8443aaa86..cab69275e473 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1515,7 +1515,14 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
>  				*force_flush = true;
>  			}
>  		}
> -		if (pte_young(ptent) && likely(vma_has_recency(vma)))
> +
> +		/*
> +		 * Skip marking exclusive file folios as accessed for processes that are
> +		 * exiting or have been reaped due to OOM. This prevents unnecessary
> +		 * promotion of folios that won't benefit the new process being launched.
> +		 */
> +		if (!exclusive_folio_of_dying_process(folio, vma) && pte_young(ptent) &&
> +				likely(vma_has_recency(vma)))
>  			folio_mark_accessed(folio);
>  		rss[mm_counter(folio)] -= nr;
>  	} else {
> -- 
> 2.39.3 (Apple Git-146)


--
Best Regards,
Yan, Zi