Re: [PATCH v3 6/9] mm/swapfile: Add batched version of folio_dup_swap

[Dev Jain]

On 11/05/26 1:15 pm, David Hildenbrand (Arm) wrote:
> On 5/6/26 11:45, Dev Jain wrote:
>> Add folio_dup_swap_pages to handle a batch of consecutive pages. Note
>> that folio_dup_swap already can handle a subset of this: nr_pages == 1 and
>> nr_pages == folio_nr_pages(folio). Generalize this to any nr_pages.
>>
>> Currently we have a not-so-nice logic of passing in subpage == NULL if
>> we mean to exercise the logic on the entire folio, and subpage != NULL if
>> we want to exercise the logic on only that subpage. Remove this
>> indirection: the caller invokes folio_dup_swap_pages() if it wants to
>> operate on a range of pages in the folio (i.e nr_pages may be anything
>> between 1 till folio_nr_pages()), and invokes folio_dup_swap() if it
>> wants to operate on the entire folio.
>>
>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>> ---
>>  mm/rmap.c     |  2 +-
>>  mm/shmem.c    |  2 +-
>>  mm/swap.h     | 12 ++++++++++--
>>  mm/swapfile.c | 20 ++++++++++++--------
>>  4 files changed, 24 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 25813e3605991..352ba77d90f67 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -2314,7 +2314,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>  				goto finish_unmap;
>>  			}
>>  
>> -			if (folio_dup_swap(folio, subpage) < 0) {
>> +			if (folio_dup_swap_pages(folio, subpage, 1) < 0) {
> 
> Can you throw in a patch to rename subpage -> page first?

Sure.

> 
>>  				set_pte_at(mm, address, pvmw.pte, pteval);
>>  				goto walk_abort;
>>  			}
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index bab3529af23c5..5e4f521399847 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1698,7 +1698,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>>  			spin_unlock(&shmem_swaplist_lock);
>>  		}
>>  
>> -		folio_dup_swap(folio, NULL);
>> +		folio_dup_swap(folio);
>>  		shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap));
>>  
>>  		BUG_ON(folio_mapped(folio));
>> diff --git a/mm/swap.h b/mm/swap.h
>> index a77016f2423b9..3c25f914e908b 100644
>> --- a/mm/swap.h
>> +++ b/mm/swap.h
>> @@ -206,7 +206,9 @@ extern int swap_retry_table_alloc(swp_entry_t entry, gfp_t gfp);
>>   * folio_put_swap(): does the opposite thing of folio_dup_swap().
>>   */
>>  int folio_alloc_swap(struct folio *folio);
>> -int folio_dup_swap(struct folio *folio, struct page *subpage);
>> +int folio_dup_swap(struct folio *folio);
>> +int folio_dup_swap_pages(struct folio *folio, struct page *page,
>> +			 unsigned long nr_pages);
>>  void folio_put_swap(struct folio *folio, struct page *subpage);
>>  
>>  /* For internal use */
>> @@ -390,7 +392,13 @@ static inline int folio_alloc_swap(struct folio *folio)
>>  	return -EINVAL;
>>  }
>>  
>> -static inline int folio_dup_swap(struct folio *folio, struct page *page)
>> +static inline int folio_dup_swap(struct folio *folio)
>> +{
>> +	return -EINVAL;
>> +}
>> +
> 
> Much cleaner.
> 
> Can we have a common folio_dup_swap() function instead that does what the
> variant below does? (call folio_dup_swap_pages())

I am guessing you are saying to simply remove the folio_dup_swap stub because
we have a stub for folio_dup_swap_pages anyways, I'll do that.

> 
>> +static inline int folio_dup_swap_pages(struct folio *folio, struct page *page,
>> +		unsigned long nr_pages)
>>  {
>>  	return -EINVAL;
>>  }
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index c7e173b93e11d..28daf92839e77 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1740,9 +1740,10 @@ int folio_alloc_swap(struct folio *folio)
>>  }
>>  
>>  /**
>> - * folio_dup_swap() - Increase swap count of swap entries of a folio.
>> + * folio_dup_swap_pages() - Increase swap count of swap entries of a folio.
>>   * @folio: folio with swap entries bounded.
>> - * @subpage: if not NULL, only increase the swap count of this subpage.
>> + * @page: the first page in the folio to increase the swap count for.
>> + * @nr_pages: the number of pages in the folio to increase the swap count for.
>>   *
>>   * Typically called when the folio is unmapped and have its swap entry to
>>   * take its place: Swap entries allocated to a folio has count == 0 and pinned
>> @@ -1756,23 +1757,26 @@ int folio_alloc_swap(struct folio *folio)
>>   * swap_put_entries_direct on its swap entry before this helper returns, or
>>   * the swap count may underflow.
>>   */
>> -int folio_dup_swap(struct folio *folio, struct page *subpage)
>> +int folio_dup_swap_pages(struct folio *folio, struct page *page,
>> +		unsigned long nr_pages)
>>  {
>>  	swp_entry_t entry = folio->swap;
>> -	unsigned long nr_pages = folio_nr_pages(folio);
>>  
>>  	VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
>>  	VM_WARN_ON_FOLIO(!folio_test_swapcache(folio), folio);
>>  
>> -	if (subpage) {
>> -		entry.val += folio_page_idx(folio, subpage);
>> -		nr_pages = 1;
>> -	}
>> +	entry.val += folio_page_idx(folio, page);
>>  
>>  	return swap_dup_entries_cluster(swap_entry_to_info(entry),
>>  					swp_offset(entry), nr_pages);
>>  }
>>  
>> +int folio_dup_swap(struct folio *folio)
>> +{
>> +	return folio_dup_swap_pages(folio, folio_page(folio, 0),
>> +				    folio_nr_pages(folio));
>> +}
> 
> Can you add simplistic kerneldoc for folio_dup_swap() as well, and mostly just
> link to folio_dup_swap_pages() ?

Okay.

>