Re: [RFC PATCH v3 3/5] mm: swap: make should_try_to_free_swap() support large-folio

[Chuanhua Han]

hi, Ryan Roberts

在 2024/3/12 20:34, Ryan Roberts 写道:
> On 04/03/2024 08:13, Barry Song wrote:
>> From: Chuanhua Han <hanchuanhua@xxxxxxxx>
>>
>> should_try_to_free_swap() works with an assumption that swap-in is always done
>> at normal page granularity, aka, folio_nr_pages = 1. To support large folio
>> swap-in, this patch removes the assumption.
>>
>> Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
>> Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
>> Acked-by: Chris Li <chrisl@xxxxxxxxxx>
>> ---
>>  mm/memory.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index abd4f33d62c9..e0d34d705e07 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -3837,7 +3837,7 @@ static inline bool should_try_to_free_swap(struct folio *folio,
>>  	 * reference only in case it's likely that we'll be the exlusive user.
>>  	 */
>>  	return (fault_flags & FAULT_FLAG_WRITE) && !folio_test_ksm(folio) &&
>> -		folio_ref_count(folio) == 2;
>> +		folio_ref_count(folio) == (1 + folio_nr_pages(folio));
> I don't think this is correct; one reference has just been added to the folio in
> do_swap_page(), either by getting from swapcache (swap_cache_get_folio()) or by
> allocating. If it came from the swapcache, it could be a large folio, because we
> swapped out a large folio and never removed it from swapcache. But in that case,
> others may have partially mapped it, so the refcount could legitimately equal
> the number of pages while still not being exclusively mapped.
>
> I'm guessing this logic is trying to estimate when we are likely exclusive so
> that we remove from swapcache (release ref) and can then reuse rather than CoW
> the folio? The main CoW path currently CoWs page-by-page even for large folios,
> and with Barry's recent patch, even the last page gets copied. So not sure what
> this change is really trying to achieve?
>
First, if it is a large folio in the swap cache, then its refcont is at
least folio_nr_pages(folio) :  


For example, in add_to_swap_cache path:

int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
                        gfp_t gfp, void **shadowp)
{
        struct address_space *address_space = swap_address_space(entry);
        pgoff_t idx = swp_offset(entry);
        XA_STATE_ORDER(xas, &address_space->i_pages, idx,
folio_order(folio));
        unsigned long i, nr = folio_nr_pages(folio); <---
        void *old;
        ...
        folio_ref_add(folio, nr); <---
        folio_set_swapcache(folio);
        ...
}


  *

    Then in the do_swap_page path:

  * if (should_try_to_free_swap(folio, vma, vmf->flags))
            folio_free_swap(folio);
  *

  * It also indicates that only folio in the swap cache will call
    folio_free_swap
  * to delete it from the swap cache, So I feel like this patch is
    necessary!? 😁

>>  }
>>  
>>  static vm_fault_t pte_marker_clear(struct vm_fault *vmf)

Thanks,

Chuanhua