Re: [PATCH v2 4/5] mm: swap: entirely map large folios found in swapcache

[Barry Song]

On Fri, Apr 12, 2024 at 3:33 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 09/04/2024 09:26, 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>
> > ---
> >  mm/memory.c | 64 +++++++++++++++++++++++++++++++++++++++++++----------
> >  1 file changed, 52 insertions(+), 12 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index c4a52e8d740a..9818dc1893c8 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3947,6 +3947,10 @@ 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 start_address = vmf->address;
> > +     pte_t *start_pte = vmf->pte;
>
> possible bug?: there are code paths that assign to vmf-pte below in this
> function, so couldn't start_pte be stale in some cases? I'd just do the
> assignment (all 4 of these variables in fact) in an else clause below, after any
> messing about with them is complete.
>
> nit: rename start_pte -> start_ptep ?

Agreed.

>
> > +     bool any_swap_shared = false;
>
> Suggest you defer initialization of this to your "We hit large folios in
> swapcache" block below, and init it to:
>
>         any_swap_shared = !pte_swp_exclusive(vmf->pte);
>
> Then the any_shared semantic in swap_pte_batch() can be the same as for
> folio_pte_batch().
>
> >
> >       if (!pte_unmap_same(vmf))
> >               goto out;
> > @@ -4137,6 +4141,35 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >        */
> >       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> >                       &vmf->ptl);
>
> bug: vmf->pte may be NULL and you are not checking it until check_pte:. Byt you
> are using it in this block. It also seems odd to do all the work in the below
> block under the PTL but before checking if the pte has changed. Suggest moving
> both checks here.

agreed.

>
> > +
> > +     /* We hit large folios in swapcache */
> > +     if (start_pte && folio_test_large(folio) && folio_test_swapcache(folio)) {
>
> What's the start_pte check protecting?

This is exactly protecting the case vmf->pte==NULL but for some reason it was
assigned in the beginning of the function incorrectly. The intention of the code
was actually doing start_pte = vmf->pte after "vmf->pte = pte_offset_map_lock".

>
> > +             int nr = folio_nr_pages(folio);
> > +             int 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_pte;
> > +             if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end)))
> > +                     goto check_pte;
> > +
> > +             folio_ptep = vmf->pte - idx;
> > +             folio_pte = ptep_get(folio_ptep);
> > +             if (!is_swap_pte(folio_pte) || non_swap_entry(pte_to_swp_entry(folio_pte)) ||
> > +                 swap_pte_batch(folio_ptep, nr, folio_pte, &any_swap_shared) != nr)
> > +                     goto check_pte;
> > +
> > +             start_address = folio_start;
> > +             start_pte = folio_ptep;
> > +             nr_pages = nr;
> > +             entry = folio->swap;
> > +             page = &folio->page;
> > +     }
> > +
> > +check_pte:
> >       if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
> >               goto out_nomap;
> >
> > @@ -4190,6 +4223,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >                        */
> >                       exclusive = false;
> >               }
> > +
> > +             /* Reuse the whole large folio iff all entries are exclusive */
> > +             if (nr_pages > 1 && any_swap_shared)
> > +                     exclusive = false;
>
> If you init any_shared with the firt pte as I suggested then you could just set
> exclusive = !any_shared at the top of this if block without needing this
> separate fixup.

Since your swap_pte_batch() function checks that all PTEs have the same
exclusive bits, I'll be removing any_shared first in version 3 per David's
suggestions. We could potentially develop "any_shared" as an incremental
patchset later on .

> >       }
> >
> >       /*
> > @@ -4204,12 +4241,14 @@ 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(entry);
> > +     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);
> >
> >       /*
> > @@ -4219,33 +4258,34 @@ 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))) {
> >               if (vmf->flags & FAULT_FLAG_WRITE) {
> >                       pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> >                       vmf->flags &= ~FAULT_FLAG_WRITE;
> >               }
> >               rmap_flags |= RMAP_EXCLUSIVE;
> >       }
> > -     flush_icache_page(vma, page);
> > +     flush_icache_pages(vma, page, nr_pages);
> >       if (pte_swp_soft_dirty(vmf->orig_pte))
> >               pte = pte_mksoft_dirty(pte);
> >       if (pte_swp_uffd_wp(vmf->orig_pte))
> >               pte = pte_mkuffd_wp(pte);
>
> I'm not sure about all this... you are smearing these SW bits from the faulting
> PTE across all the ptes you are mapping. Although I guess actually that's ok
> because swap_pte_batch() only returns a batch with all these bits the same?

Initially, I didn't recognize the issue at all because the tested
architecture arm64
didn't include these bits. However, after reviewing your latest swpout series,
which verifies the consistent bits for soft_dirty and uffd_wp, I now
feel  its safety
even for platforms with these bits.

>
> > -     vmf->orig_pte = pte;
>
> Instead of doing a readback below, perhaps:
>
>         vmf->orig_pte = pte_advance_pfn(pte, nr_pages);

Nice !

>
> >
> >       /* ksm created a completely new copy */
> >       if (unlikely(folio != swapcache && swapcache)) {
> > -             folio_add_new_anon_rmap(folio, vma, vmf->address);
> > +             folio_add_new_anon_rmap(folio, vma, start_address);
> >               folio_add_lru_vma(folio, vma);
> >       } else {
> > -             folio_add_anon_rmap_pte(folio, page, vma, vmf->address,
> > -                                     rmap_flags);
> > +             folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, start_address,
> > +                                      rmap_flags);
> >       }
> >
> >       VM_BUG_ON(!folio_test_anon(folio) ||
> >                       (pte_write(pte) && !PageAnonExclusive(page)));
> > -     set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
> > -     arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
> > +     set_ptes(vma->vm_mm, start_address, start_pte, pte, nr_pages);
> > +     vmf->orig_pte = ptep_get(vmf->pte);
> > +     arch_do_swap_page(vma->vm_mm, vma, start_address, pte, pte);
> >
> >       folio_unlock(folio);
> >       if (folio != swapcache && swapcache) {
> > @@ -4269,7 +4309,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       }
> >
> >       /* No need to invalidate - it was non-present before */
> > -     update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
> > +     update_mmu_cache_range(vmf, vma, start_address, start_pte, nr_pages);
> >  unlock:
> >       if (vmf->pte)
> >               pte_unmap_unlock(vmf->pte, vmf->ptl);
>

Thanks
Barry