Re: [PATCH RFC 4/6] mm: support large folios swapin as a whole

[Barry Song <21cnbao@gmail.com>]

On Sun, Jan 28, 2024 at 9:06 AM Chris Li <chrisl@kernel.org> wrote:
>
> On Thu, Jan 18, 2024 at 3:12 AM Barry Song <21cnbao@gmail.com> wrote:
> >
> > From: Chuanhua Han <hanchuanhua@oppo.com>
> >
> > On an embedded system like Android, more than half of anon memory is actually
> > in swap devices such as zRAM. For example, while an app is switched to back-
> > ground, its most memory might be swapped-out.
> >
> > Now we have mTHP features, unfortunately, if we don't support large folios
> > swap-in, once those large folios are swapped-out, we immediately lose the
> > performance gain we can get through large folios and hardware optimization
> > such as CONT-PTE.
> >
> > This patch brings up mTHP swap-in support. Right now, we limit mTHP swap-in
> > to those contiguous swaps which were likely swapped out from mTHP as a whole.
> >
> > On the other hand, the current implementation only covers the SWAP_SYCHRONOUS
> > case. It doesn't support swapin_readahead as large folios yet.
> >
> > Right now, we are re-faulting large folios which are still in swapcache as a
> > whole, this can effectively decrease extra loops and early-exitings which we
> > have increased in arch_swap_restore() while supporting MTE restore for folios
> > rather than page.
> >
> > Signed-off-by: Chuanhua Han <hanchuanhua@oppo.com>
> > Co-developed-by: Barry Song <v-songbaohua@oppo.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >  mm/memory.c | 108 +++++++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 94 insertions(+), 14 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index f61a48929ba7..928b3f542932 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -107,6 +107,8 @@ EXPORT_SYMBOL(mem_map);
> >  static vm_fault_t do_fault(struct vm_fault *vmf);
> >  static vm_fault_t do_anonymous_page(struct vm_fault *vmf);
> >  static bool vmf_pte_changed(struct vm_fault *vmf);
> > +static struct folio *alloc_anon_folio(struct vm_fault *vmf,
> > +                                     bool (*pte_range_check)(pte_t *, int));
> >
> >  /*
> >   * Return true if the original pte was a uffd-wp pte marker (so the pte was
> > @@ -3784,6 +3786,34 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> >         return VM_FAULT_SIGBUS;
> >  }
> >
> > +static bool pte_range_swap(pte_t *pte, int nr_pages)
> > +{
> > +       int i;
> > +       swp_entry_t entry;
> > +       unsigned type;
> > +       pgoff_t start_offset;
> > +
> > +       entry = pte_to_swp_entry(ptep_get_lockless(pte));
> > +       if (non_swap_entry(entry))
> > +               return false;
> > +       start_offset = swp_offset(entry);
> > +       if (start_offset % nr_pages)
> > +               return false;
> > +
> > +       type = swp_type(entry);
> > +       for (i = 1; i < nr_pages; i++) {
> > +               entry = pte_to_swp_entry(ptep_get_lockless(pte + i));
> > +               if (non_swap_entry(entry))
> > +                       return false;
> > +               if (swp_offset(entry) != start_offset + i)
> > +                       return false;
> > +               if (swp_type(entry) != type)
> > +                       return false;
> > +       }
> > +
> > +       return true;
> > +}
> > +
> >  /*
> >   * We enter with non-exclusive mmap_lock (to exclude vma changes,
> >   * but allow concurrent faults), and pte mapped but not yet locked.
> > @@ -3804,6 +3834,9 @@ 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;
> > +       pte_t *start_pte;
> >
> >         if (!pte_unmap_same(vmf))
> >                 goto out;
> > @@ -3868,13 +3901,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >                 if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> >                     __swap_count(entry) == 1) {
> >                         /* skip swapcache */
> > -                       folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> > -                                               vma, vmf->address, false);
> > +                       folio = alloc_anon_folio(vmf, pte_range_swap);
> >                         page = &folio->page;
> >                         if (folio) {
> >                                 __folio_set_locked(folio);
> >                                 __folio_set_swapbacked(folio);
> >
> > +                               if (folio_test_large(folio)) {
> > +                                       unsigned long start_offset;
> > +
> > +                                       nr_pages = folio_nr_pages(folio);
> > +                                       start_offset = swp_offset(entry) & ~(nr_pages - 1);
> > +                                       entry = swp_entry(swp_type(entry), start_offset);
> > +                               }
> > +
> >                                 if (mem_cgroup_swapin_charge_folio(folio,
> >                                                         vma->vm_mm, GFP_KERNEL,
> >                                                         entry)) {
> > @@ -3980,6 +4020,39 @@ 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);
> > +
> > +       start_address = vmf->address;
> > +       start_pte = vmf->pte;
> > +       if (folio_test_large(folio)) {
> > +               unsigned long nr = folio_nr_pages(folio);
> > +               unsigned long addr = ALIGN_DOWN(vmf->address, nr * PAGE_SIZE);
> > +               pte_t *pte_t = vmf->pte - (vmf->address - addr) / PAGE_SIZE;
>
> I forgot about one comment here.
> Please change the variable name other than "pte_t", it is a bit
> strange to use the typedef name as variable name here.
>

make sense!

> Chris

Thanks
Barry