Re: [PATCH v1 10/10] mm: Allocate large folios for anonymous memory

[Yang Shi]

On Thu, Jun 29, 2023 at 4:30 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 29/06/2023 03:13, Yang Shi wrote:
> > On Mon, Jun 26, 2023 at 10:15 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> >>
> >> With all of the enabler patches in place, modify the anonymous memory
> >> write allocation path so that it opportunistically attempts to allocate
> >> a large folio up to `max_anon_folio_order()` size (This value is
> >> ultimately configured by the architecture). This reduces the number of
> >> page faults, reduces the size of (e.g. LRU) lists, and generally
> >> improves performance by batching what were per-page operations into
> >> per-(large)-folio operations.
> >>
> >> If CONFIG_LARGE_ANON_FOLIO is not enabled (the default) then
> >> `max_anon_folio_order()` always returns 0, meaning we get the existing
> >> allocation behaviour.
> >>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >> ---
> >>  mm/memory.c | 159 +++++++++++++++++++++++++++++++++++++++++++++++-----
> >>  1 file changed, 144 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index a8f7e2b28d7a..d23c44cc5092 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -3161,6 +3161,90 @@ static inline int max_anon_folio_order(struct vm_area_struct *vma)
> >>                 return CONFIG_LARGE_ANON_FOLIO_NOTHP_ORDER_MAX;
> >>  }
> >>
> >> +/*
> >> + * Returns index of first pte that is not none, or nr if all are none.
> >> + */
> >> +static inline int check_ptes_none(pte_t *pte, int nr)
> >> +{
> >> +       int i;
> >> +
> >> +       for (i = 0; i < nr; i++) {
> >> +               if (!pte_none(ptep_get(pte++)))
> >> +                       return i;
> >> +       }
> >> +
> >> +       return nr;
> >> +}
> >> +
> >> +static int calc_anon_folio_order_alloc(struct vm_fault *vmf, int order)
> >> +{
> >> +       /*
> >> +        * The aim here is to determine what size of folio we should allocate
> >> +        * for this fault. Factors include:
> >> +        * - Order must not be higher than `order` upon entry
> >> +        * - Folio must be naturally aligned within VA space
> >> +        * - Folio must not breach boundaries of vma
> >> +        * - Folio must be fully contained inside one pmd entry
> >> +        * - Folio must not overlap any non-none ptes
> >> +        *
> >> +        * Additionally, we do not allow order-1 since this breaks assumptions
> >> +        * elsewhere in the mm; THP pages must be at least order-2 (since they
> >> +        * store state up to the 3rd struct page subpage), and these pages must
> >> +        * be THP in order to correctly use pre-existing THP infrastructure such
> >> +        * as folio_split().
> >> +        *
> >> +        * As a consequence of relying on the THP infrastructure, if the system
> >> +        * does not support THP, we always fallback to order-0.
> >> +        *
> >> +        * Note that the caller may or may not choose to lock the pte. If
> >> +        * unlocked, the calculation should be considered an estimate that will
> >> +        * need to be validated under the lock.
> >> +        */
> >> +
> >> +       struct vm_area_struct *vma = vmf->vma;
> >> +       int nr;
> >> +       unsigned long addr;
> >> +       pte_t *pte;
> >> +       pte_t *first_set = NULL;
> >> +       int ret;
> >> +
> >> +       if (has_transparent_hugepage()) {
> >> +               order = min(order, PMD_SHIFT - PAGE_SHIFT);
> >> +
> >> +               for (; order > 1; order--) {
> >> +                       nr = 1 << order;
> >> +                       addr = ALIGN_DOWN(vmf->address, nr << PAGE_SHIFT);
> >> +                       pte = vmf->pte - ((vmf->address - addr) >> PAGE_SHIFT);
> >> +
> >> +                       /* Check vma bounds. */
> >> +                       if (addr < vma->vm_start ||
> >> +                           addr + (nr << PAGE_SHIFT) > vma->vm_end)
> >> +                               continue;
> >> +
> >> +                       /* Ptes covered by order already known to be none. */
> >> +                       if (pte + nr <= first_set)
> >> +                               break;
> >> +
> >> +                       /* Already found set pte in range covered by order. */
> >> +                       if (pte <= first_set)
> >> +                               continue;
> >> +
> >> +                       /* Need to check if all the ptes are none. */
> >> +                       ret = check_ptes_none(pte, nr);
> >> +                       if (ret == nr)
> >> +                               break;
> >> +
> >> +                       first_set = pte + ret;
> >> +               }
> >> +
> >> +               if (order == 1)
> >> +                       order = 0;
> >> +       } else
> >> +               order = 0;
> >> +
> >> +       return order;
> >> +}
> >> +
> >>  /*
> >>   * Handle write page faults for pages that can be reused in the current vma
> >>   *
> >> @@ -4201,6 +4285,9 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >>         struct folio *folio;
> >>         vm_fault_t ret = 0;
> >>         pte_t entry;
> >> +       unsigned long addr;
> >> +       int order = uffd_wp ? 0 : max_anon_folio_order(vma);
> >> +       int pgcount = BIT(order);
> >>
> >>         /* File mapping without ->vm_ops ? */
> >>         if (vma->vm_flags & VM_SHARED)
> >> @@ -4242,24 +4329,44 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >>                         pte_unmap_unlock(vmf->pte, vmf->ptl);
> >>                         return handle_userfault(vmf, VM_UFFD_MISSING);
> >>                 }
> >> -               goto setpte;
> >> +               if (uffd_wp)
> >> +                       entry = pte_mkuffd_wp(entry);
> >> +               set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
> >> +
> >> +               /* No need to invalidate - it was non-present before */
> >> +               update_mmu_cache(vma, vmf->address, vmf->pte);
> >> +               goto unlock;
> >>         }
> >>
> >> -       /* Allocate our own private page. */
> >> +retry:
> >> +       /*
> >> +        * Estimate the folio order to allocate. We are not under the ptl here
> >> +        * so this estiamte needs to be re-checked later once we have the lock.
> >> +        */
> >> +       vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
> >> +       order = calc_anon_folio_order_alloc(vmf, order);
> >> +       pte_unmap(vmf->pte);
> >> +
> >> +       /* Allocate our own private folio. */
> >>         if (unlikely(anon_vma_prepare(vma)))
> >>                 goto oom;
> >> -       folio = vma_alloc_zeroed_movable_folio(vma, vmf->address, 0, 0);
> >> +       folio = try_vma_alloc_movable_folio(vma, vmf->address, order, true);
> >>         if (!folio)
> >>                 goto oom;
> >>
> >> +       /* We may have been granted less than we asked for. */
> >> +       order = folio_order(folio);
> >> +       pgcount = BIT(order);
> >> +       addr = ALIGN_DOWN(vmf->address, pgcount << PAGE_SHIFT);
> >> +
> >>         if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
> >>                 goto oom_free_page;
> >>         folio_throttle_swaprate(folio, GFP_KERNEL);
> >>
> >>         /*
> >>          * The memory barrier inside __folio_mark_uptodate makes sure that
> >> -        * preceding stores to the page contents become visible before
> >> -        * the set_pte_at() write.
> >> +        * preceding stores to the folio contents become visible before
> >> +        * the set_ptes() write.
> >>          */
> >>         __folio_mark_uptodate(folio);
> >>
> >> @@ -4268,11 +4375,31 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >>         if (vma->vm_flags & VM_WRITE)
> >>                 entry = pte_mkwrite(pte_mkdirty(entry));
> >>
> >> -       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> >> -                       &vmf->ptl);
> >> -       if (vmf_pte_changed(vmf)) {
> >> -               update_mmu_tlb(vma, vmf->address, vmf->pte);
> >> -               goto release;
> >> +       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
> >> +
> >> +       /*
> >> +        * Ensure our estimate above is still correct; we could have raced with
> >> +        * another thread to service a fault in the region.
> >> +        */
> >> +       if (order == 0) {
> >> +               if (vmf_pte_changed(vmf)) {
> >> +                       update_mmu_tlb(vma, vmf->address, vmf->pte);
> >> +                       goto release;
> >> +               }
> >> +       } else if (check_ptes_none(vmf->pte, pgcount) != pgcount) {
> >> +               pte_t *pte = vmf->pte + ((vmf->address - addr) >> PAGE_SHIFT);
> >> +
> >> +               /* If faulting pte was allocated by another, exit early. */
> >> +               if (!pte_none(ptep_get(pte))) {
> >> +                       update_mmu_tlb(vma, vmf->address, pte);
> >> +                       goto release;
> >> +               }
> >> +
> >> +               /* Else try again, with a lower order. */
> >> +               pte_unmap_unlock(vmf->pte, vmf->ptl);
> >> +               folio_put(folio);
> >> +               order--;
> >> +               goto retry;
> >
> > I'm not sure whether this extra fallback logic is worth it or not. Do
> > you have any benchmark data or is it just an arbitrary design choice?
> > If it is just an arbitrary design choice, I'd like to go with the
> > simplest way by just exiting page fault handler, just like the
> > order-0, IMHO.
>
> Yes, its an arbitrary design choice. Based on Yu Zhao's feedback, I'm already
> reworking this so that we only try the preferred order and order-0, so no longer
> iterating through intermediate orders.
>
> I think what you are suggesting is that if attempting to allocate the preferred
> order and we find there was a race meaning that the folio now is overlapping
> populated ptes (but the faulting pte is still empty), just exit and rely on the
> page fault being re-triggered, rather than immediately falling back to order-0?

The faulting PTE might be filled too. Yes, just exit and rely on the
CPU re-trigger page fault.

>
> The reason I didn't do that was I wasn't sure if the return path might have
> assumptions that the faulting pte is now valid if no error was returned? I guess
> another option is to return VM_FAULT_RETRY but then it seemed cleaner to do the
> retry directly here. What do you suggest?

IIRC as long as the page fault handler doesn't return any error, it is
safe to rely on CPU re-trigger page fault if PTE is not installed.

VM_FAULT_RETRY means the page fault handler released mmap_lock (or
per-VMA lock with per-VMA lock enabled) due to waiting for page lock.
TBH I really don't want to make that semantic more complicated and
overloaded. And I don't see any fundamental difference between
vmf_pte_changed() for order-0 folio and overlapping PTEs installed for
large folio. So I'd like to follow the same behavior.

>
> Thanks,
> Ryan
>
>
>
> >
> >>         }
> >>
> >>         ret = check_stable_address_space(vma->vm_mm);
> >> @@ -4286,16 +4413,18 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >>                 return handle_userfault(vmf, VM_UFFD_MISSING);
> >>         }
> >>
> >> -       inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> >> -       folio_add_new_anon_rmap(folio, vma, vmf->address);
> >> +       folio_ref_add(folio, pgcount - 1);
> >> +
> >> +       add_mm_counter(vma->vm_mm, MM_ANONPAGES, pgcount);
> >> +       folio_add_new_anon_rmap_range(folio, &folio->page, pgcount, vma, addr);
> >>         folio_add_lru_vma(folio, vma);
> >> -setpte:
> >> +
> >>         if (uffd_wp)
> >>                 entry = pte_mkuffd_wp(entry);
> >> -       set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
> >> +       set_ptes(vma->vm_mm, addr, vmf->pte, entry, pgcount);
> >>
> >>         /* No need to invalidate - it was non-present before */
> >> -       update_mmu_cache(vma, vmf->address, vmf->pte);
> >> +       update_mmu_cache_range(vma, addr, vmf->pte, pgcount);
> >>  unlock:
> >>         pte_unmap_unlock(vmf->pte, vmf->ptl);
> >>         return ret;
> >> --
> >> 2.25.1
> >>
> >>
>