Re: [PATCH 6/9] mm/swapfile: Make folio_dup_swap batchable

[Dev Jain]

On 10/03/26 2:19 pm, Lorenzo Stoakes (Oracle) wrote:
> On Tue, Mar 10, 2026 at 01:00:10PM +0530, Dev Jain wrote:
>> Teach folio_dup_swap 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, and explicitly pass subpage != NULL, and the number of
>> pages required.
> 
> You've made the interface more confusing? Now we can update multiple subpages
> but specify only one? :)
> 
> Let's try to actually refactor this into something sane... see below.
> 
>>
>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>> ---
>>  mm/rmap.c     |  2 +-
>>  mm/shmem.c    |  2 +-
>>  mm/swap.h     |  5 +++--
>>  mm/swapfile.c | 12 +++++-------
>>  4 files changed, 10 insertions(+), 11 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index dd638429c963e..f6d5b187cf09b 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -2282,7 +2282,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>  				goto discard;
>>  			}
>>
>> -			if (folio_dup_swap(folio, subpage) < 0) {
>> +			if (folio_dup_swap(folio, subpage, 1) < 0) {
>>  				set_pte_at(mm, address, pvmw.pte, pteval);
>>  				goto walk_abort;
>>  			}
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 5e7dcf5bc5d3c..86ee34c9b40b3 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1695,7 +1695,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>>  			spin_unlock(&shmem_swaplist_lock);
>>  		}
>>
>> -		folio_dup_swap(folio, NULL);
>> +		folio_dup_swap(folio, folio_page(folio, 0), folio_nr_pages(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..d9cb58ebbddd1 100644
>> --- a/mm/swap.h
>> +++ b/mm/swap.h
>> @@ -206,7 +206,7 @@ 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, struct page *subpage, unsigned int nr_pages);
>>  void folio_put_swap(struct folio *folio, struct page *subpage);
>>
>>  /* For internal use */
>> @@ -390,7 +390,8 @@ 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, struct page *page,
>> +				 unsigned int nr_pages)
>>  {
>>  	return -EINVAL;
>>  }
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 915bc93964dbd..eaf61ae6c3817 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1738,7 +1738,8 @@ int folio_alloc_swap(struct folio *folio)
>>  /**
>>   * folio_dup_swap() - 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.
>> + * @subpage: Increase the swap count of this subpage till nr number of
>> + * pages forward.
> 
> (Obviously also Kairui's point about missing entry in kdoc)
> 
> This is REALLY confusing sorry. And this interface is just a horror show.
> 
> Before we had subpage == only increase the swap count of the subpage.
> 
> Now subpage = the first subpage at which we do that? Please, no.
> 
> You just need to rework this interface in general, this is a hack.
> 
> Something like:
> 
> int __folio_dup_swap(struct folio *folio, unsigned int subpage_start_index,
> 		     unsigned int nr_subpages)
> {
> 	...
> }
> 
> ...
> 
> int folio_dup_swap_subpage(struct folio *folio, struct page *subpage)
> {
> 	return __folio_dup_swap(folio, folio_page_idx(folio, subpage), 1);
> }
> 
> int folio_dup_swap(struct folio *folio)
> {
> 	return __folio_dup_swap(folio, 0, folio_nr_pages(folio));
> }
> 
> Or something like that.


I get the essence of the point you are making.

Since most callers of folio_put_swap mean it for entire folio, perhaps
we can have folio_put_swap for these callers, and the ones which are
not sure can call folio_put_swap_subpages? Same for folio_dup_swap.
And since we are calling it folio_put_swap_subpages, we can retain
the subpage parameter?

> 
> We're definitely _not_ keeping the subpage parameter like that and hacking on
> batching, PLEASE.
> 
>>   *
>>   * 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
>> @@ -1752,18 +1753,15 @@ 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(struct folio *folio, struct page *subpage,
>> +		   unsigned int 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, subpage);
>>
>>  	return swap_dup_entries_cluster(swap_entry_to_info(entry),
>>  					swp_offset(entry), nr_pages);
>> --
>> 2.34.1
>>
> 
> Thanks, Lorenzo