Re: [RFC 1/3] mm: process_mrelease: expedite clean file folio reclaim via mmu_gather

[David Hildenbrand (Arm)]

On 4/14/26 00:39, Minchan Kim wrote:
> Currently, process_mrelease() unmaps the pages but leaves clean file
> folios on the LRU list, relying on standard memory reclaim to eventually
> free them. This delays the immediate recovery of system memory under OOM
> or container shutdown scenarios.

process_mrelease() calls __oom_reap_task_mm().

There, we skip any MAP_SHARED file mappings.

So I assume what you describe only applies to MAP_PRIVATE file mappings?
What about MAP_SHARED?

Also "leaves ... on the LRU list" is rather confusing. They are not
evicted and stay in the pagecache?

> 
> This patch implements an expedited eviction mechanism for clean file
> folios by integrating directly into the low-level TLB batching
> infrastructure (mmu_gather).

Is this a complicated way of saying "Handle clean pagecache folios
similar to swapcache folios in mmu_gather code, dropping them from the
swapcache (i.e., evicting them) if they are completely unmapped during
reaping"?

> 
> Instead of repeatedly locking and evicting folios one by one inside the
> unmap loop (zap_present_folio_ptes), we pass the MMF_UNSTABLE flag
> status down to free_pages_and_swap_cache(). Within this single unified
> loop, anonymous pages are released via free_swap_cache(), and
> file-backed folios are symmetrically truncated via mapping_evict_folio().

... where you still evict them one-by-one. Rather confusing.

> 
> This avoids introducing unnecessary data structures, preserves TLB flush
> safety, and removes duplicate tree traversals, resulting in an extremely
> lean and highly responsive process_mrelease() implementation.

I don't think this paragraph adds a lot of value, really.

Which "duplicate tree traversal"? Which unnecessary data structures?

Is that AI generated text? A lot of the stuff here reads AI generated. I
yet have to meet a developer (not a sales person) that would just say
"extremely lean and highly responsive process_mrelease() implementation"

If it is AI generated, throw it away and write it yourself from scratch.
Use AI only to polish your English.

> 
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>  arch/s390/include/asm/tlb.h |  2 +-
>  include/linux/swap.h        |  9 ++++++---
>  mm/mmu_gather.c             |  8 +++++---
>  mm/swap_state.c             | 19 +++++++++++++++++--
>  4 files changed, 29 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/s390/include/asm/tlb.h b/arch/s390/include/asm/tlb.h
> index 619fd41e710e..554842345ccd 100644
> --- a/arch/s390/include/asm/tlb.h
> +++ b/arch/s390/include/asm/tlb.h
> @@ -62,7 +62,7 @@ static inline bool __tlb_remove_folio_pages(struct mmu_gather *tlb,
>  	VM_WARN_ON_ONCE(delay_rmap);
>  	VM_WARN_ON_ONCE(page_folio(page) != page_folio(page + nr_pages - 1));
>  
> -	free_pages_and_swap_cache(encoded_pages, ARRAY_SIZE(encoded_pages));
> +	free_pages_and_caches(encoded_pages, ARRAY_SIZE(encoded_pages), false);

As we dislike boolean parameters, we either try to avoid them (e.g., use
flags) or document the parameters using something like

"/* parameter_name= */false"

>  	return false;
>  }
>  
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 62fc7499b408..e7b929b062f8 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -433,7 +433,7 @@ static inline unsigned long total_swapcache_pages(void)
>  
>  void free_swap_cache(struct folio *folio);
>  void free_folio_and_swap_cache(struct folio *folio);
> -void free_pages_and_swap_cache(struct encoded_page **, int);
> +void free_pages_and_caches(struct encoded_page **pages, int nr, bool free_unmapped_file);
>  /* linux/mm/swapfile.c */
>  extern atomic_long_t nr_swap_pages;
>  extern long total_swap_pages;
> @@ -510,8 +510,11 @@ static inline void put_swap_device(struct swap_info_struct *si)
>  	do { (val)->freeswap = (val)->totalswap = 0; } while (0)
>  #define free_folio_and_swap_cache(folio) \
>  	folio_put(folio)
> -#define free_pages_and_swap_cache(pages, nr) \
> -	release_pages((pages), (nr));
> +static inline void free_pages_and_caches(struct encoded_page **pages,
> +		int nr, bool free_unmapped_file)
> +{
> +	release_pages(pages, nr);
> +}

Why should !CONFIG_SWAP not take care of free_unmapped_file?


>  
>  static inline void free_swap_cache(struct folio *folio)
>  {
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index fe5b6a031717..5ce5824db07f 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -100,7 +100,8 @@ void tlb_flush_rmaps(struct mmu_gather *tlb, struct vm_area_struct *vma)
>   */
>  #define MAX_NR_FOLIOS_PER_FREE		512
>  
> -static void __tlb_batch_free_encoded_pages(struct mmu_gather_batch *batch)
> +static void __tlb_batch_free_encoded_pages(struct mm_struct *mm,
> +		struct mmu_gather_batch *batch)
>  {
>  	struct encoded_page **pages = batch->encoded_pages;
>  	unsigned int nr, nr_pages;
> @@ -135,7 +136,8 @@ static void __tlb_batch_free_encoded_pages(struct mmu_gather_batch *batch)
>  			}
>  		}
>  
> -		free_pages_and_swap_cache(pages, nr);
> +		free_pages_and_caches(pages, nr,
> +				      mm_flags_test(MMF_UNSTABLE, mm));
>  		pages += nr;
>  		batch->nr -= nr;
>  
> @@ -148,7 +150,7 @@ static void tlb_batch_pages_flush(struct mmu_gather *tlb)
>  	struct mmu_gather_batch *batch;
>  
>  	for (batch = &tlb->local; batch && batch->nr; batch = batch->next)
> -		__tlb_batch_free_encoded_pages(batch);
> +		__tlb_batch_free_encoded_pages(tlb->mm, batch);
>  	tlb->active = &tlb->local;
>  }
>  
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 6d0eef7470be..e70a52ead6d3 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -400,11 +400,22 @@ void free_folio_and_swap_cache(struct folio *folio)
>  		folio_put(folio);
>  }
>  
> +static inline void free_file_cache(struct folio *folio)
> +{
> +	if (folio_trylock(folio)) {
> +		mapping_evict_folio(folio_mapping(folio), folio);
> +		folio_unlock(folio);
> +	}
> +}
> +
>  /*
>   * Passed an array of pages, drop them all from swapcache and then release
>   * them.  They are removed from the LRU and freed if this is their last use.
> + *
> + * If @free_unmapped_file is true, this function will proactively evict clean
> + * file-backed folios if they are no longer mapped.

The parameter name is not really expressive.

You are not freeing unmapped files.

"try_evict_file_folios" maybe?

mapping_evict_folio() has exactly these semantics (unmapped, clean)

>   */
> -void free_pages_and_swap_cache(struct encoded_page **pages, int nr)
> +void free_pages_and_caches(struct encoded_page **pages, int nr, bool free_unmapped_file)
>  {
>  	struct folio_batch folios;
>  	unsigned int refs[PAGEVEC_SIZE];
> @@ -413,7 +424,11 @@ void free_pages_and_swap_cache(struct encoded_page **pages, int nr)
>  	for (int i = 0; i < nr; i++) {
>  		struct folio *folio = page_folio(encoded_page_ptr(pages[i]));
>  
> -		free_swap_cache(folio);
> +		if (folio_test_anon(folio))
> +			free_swap_cache(folio);
> +		else if (unlikely(free_unmapped_file))
> +			free_file_cache(folio);
> +
>  		refs[folios.nr] = 1;
>  		if (unlikely(encoded_page_flags(pages[i]) &
>  			     ENCODED_PAGE_BIT_NR_PAGES_NEXT))


-- 
Cheers,

David