On 13/03/2024 02:21, Chuanhua Han wrote:
> 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) :
Ahh! Sorry, I had it backwards - was thinking there would be 1 ref for the swap
cache, and you were assuming 1 ref per page taken by do_swap_page(). I
understand now. On this basis:
Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>
>
> 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
>
