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

[Chuanhua Han]

Hi Ryan,
Ryan Roberts <ryan.roberts@xxxxxxx> 于2024年3月13日周三 17:10写道：
>
> 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>

Thank you for your review!
>
> >
> >
> > 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
> >
>
>
Thanks,
Chuanhua