Re: [PATCH v3 6/6] mm: swap: entirely map large folios found in swapcache

[Barry Song]

On Tue, May 7, 2024 at 12:07 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 04.05.24 01:23, Barry Song wrote:
> > On Fri, May 3, 2024 at 6:50 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> >>
> >> On 03/05/2024 01:50, Barry Song wrote:
> >>> From: Chuanhua Han <hanchuanhua@xxxxxxxx>
> >>>
> >>> When a large folio is found in the swapcache, the current implementation
> >>> requires calling do_swap_page() nr_pages times, resulting in nr_pages
> >>> page faults. This patch opts to map the entire large folio at once to
> >>> minimize page faults. Additionally, redundant checks and early exits
> >>> for ARM64 MTE restoring are removed.
> >>>
> >>> Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
> >>> Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>
> >> With the suggested changes below:
> >>
> >> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >>
> >>> ---
> >>>   mm/memory.c | 60 ++++++++++++++++++++++++++++++++++++++++++-----------
> >>>   1 file changed, 48 insertions(+), 12 deletions(-)
> >>>
> >>> diff --git a/mm/memory.c b/mm/memory.c
> >>> index 22e7c33cc747..940fdbe69fa1 100644
> >>> --- a/mm/memory.c
> >>> +++ b/mm/memory.c
> >>> @@ -3968,6 +3968,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>>        pte_t pte;
> >>>        vm_fault_t ret = 0;
> >>>        void *shadow = NULL;
> >>> +     int nr_pages = 1;
> >>> +     unsigned long page_idx = 0;
> >>> +     unsigned long address = vmf->address;
> >>> +     pte_t *ptep;
> >>
> >> nit: Personally I'd prefer all these to get initialised just before the "if
> >> (folio_test_large()..." block below. That way it is clear they are fresh (incase
> >> any logic between here and there makes an adjustment) and its clear that they
> >> are only to be used after that block (the compiler will warn if using an
> >> uninitialized value).
> >
> > right. I agree this will make the code more readable.
> >
> >>
> >>>
> >>>        if (!pte_unmap_same(vmf))
> >>>                goto out;
> >>> @@ -4166,6 +4170,36 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>>                goto out_nomap;
> >>>        }
> >>>
> >>> +     ptep = vmf->pte;
> >>> +     if (folio_test_large(folio) && folio_test_swapcache(folio)) {
> >>> +             int nr = folio_nr_pages(folio);
> >>> +             unsigned long idx = folio_page_idx(folio, page);
> >>> +             unsigned long folio_start = vmf->address - idx * PAGE_SIZE;
> >>> +             unsigned long folio_end = folio_start + nr * PAGE_SIZE;
> >>> +             pte_t *folio_ptep;
> >>> +             pte_t folio_pte;
> >>> +
> >>> +             if (unlikely(folio_start < max(vmf->address & PMD_MASK, vma->vm_start)))
> >>> +                     goto check_folio;
> >>> +             if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end)))
> >>> +                     goto check_folio;
> >>> +
> >>> +             folio_ptep = vmf->pte - idx;
> >>> +             folio_pte = ptep_get(folio_ptep);
> >>> +             if (!pte_same(folio_pte, pte_move_swp_offset(vmf->orig_pte, -idx)) ||
> >>> +                 swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
> >>> +                     goto check_folio;
> >>> +
> >>> +             page_idx = idx;
> >>> +             address = folio_start;
> >>> +             ptep = folio_ptep;
> >>> +             nr_pages = nr;
> >>> +             entry = folio->swap;
> >>> +             page = &folio->page;
> >>> +     }
> >>> +
> >>> +check_folio:
> >>
> >> Is this still the correct label name, given the checks are now above the new
> >> block? Perhaps "one_page" or something like that?
> >
> > not quite sure about this, as the code after one_page can be multiple_pages.
> > On the other hand, it seems we are really checking folio after "check_folio"
> > :-)
> >
> >
> > BUG_ON(!folio_test_anon(folio) && folio_test_mappedtodisk(folio));
> > BUG_ON(folio_test_anon(folio) && PageAnonExclusive(page));
> >
> > /*
> > * Check under PT lock (to protect against concurrent fork() sharing
> > * the swap entry concurrently) for certainly exclusive pages.
> > */
> > if (!folio_test_ksm(folio)) {
> >
> >
> >>
> >>> +
> >>>        /*
> >>>         * PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte
> >>>         * must never point at an anonymous page in the swapcache that is
> >>> @@ -4225,12 +4259,13 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>>         * We're already holding a reference on the page but haven't mapped it
> >>>         * yet.
> >>>         */
> >>> -     swap_free_nr(entry, 1);
> >>> +     swap_free_nr(entry, nr_pages);
> >>>        if (should_try_to_free_swap(folio, vma, vmf->flags))
> >>>                folio_free_swap(folio);
> >>>
> >>> -     inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> >>> -     dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> >>> +     folio_ref_add(folio, nr_pages - 1);
> >>> +     add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> >>> +     add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
> >>>        pte = mk_pte(page, vma->vm_page_prot);
> >>>
> >>>        /*
> >>> @@ -4240,34 +4275,35 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>>         * exclusivity.
> >>>         */
> >>>        if (!folio_test_ksm(folio) &&
> >>> -         (exclusive || folio_ref_count(folio) == 1)) {
> >>> +         (exclusive || (folio_ref_count(folio) == nr_pages &&
> >>> +                        folio_nr_pages(folio) == nr_pages))) {
> >>
> >> I think in practice there is no change here? If nr_pages > 1 then the folio is
> >> in the swapcache, so there is an extra ref on it? I agree with the change for
> >> robustness sake. Just checking my understanding.
> >
> > This is the code showing we are reusing/(mkwrite) a folio either
> > 1. we meet a small folio and we are the only one hitting the small folio
> > 2. we meet a large folio and we are the only one hitting the large folio
> >
> > any corner cases besides the above two seems difficult. for example,
> >
> > while we hit a large folio in swapcache but if we can't entirely map it
> > (nr_pages==1) due to partial unmap, we will have folio_ref_count(folio)
> > == nr_pages == 1
>
> No, there would be other references from the swapcache and
> folio_ref_count(folio) > 1. See my other reply.

right. can be clearer by:

@@ -4263,7 +4264,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
        if (should_try_to_free_swap(folio, vma, vmf->flags))
                folio_free_swap(folio);

-       folio_ref_add(folio, nr_pages - 1);
        add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
        add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
        pte = mk_pte(page, vma->vm_page_prot);
@@ -4275,14 +4275,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
         * exclusivity.
         */
        if (!folio_test_ksm(folio) &&
-           (exclusive || (folio_ref_count(folio) == nr_pages &&
-                          folio_nr_pages(folio) == nr_pages))) {
+           (exclusive || folio_ref_count(folio) == 1)) {
                if (vmf->flags & FAULT_FLAG_WRITE) {
                        pte = maybe_mkwrite(pte_mkdirty(pte), vma);
                        vmf->flags &= ~FAULT_FLAG_WRITE;
                }
                rmap_flags |= RMAP_EXCLUSIVE;
        }
+       folio_ref_add(folio, nr_pages - 1);
        flush_icache_pages(vma, page, nr_pages);
        if (pte_swp_soft_dirty(vmf->orig_pte))
                pte = pte_mksoft_dirty(pte);


>
> --
> Cheers,
>
> David / dhildenb
>