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

Yang Shi
2023-06-29T02:13:29Z

On Mon, Jun 26, 2023 at 10:15 AM Ryan Roberts <ryan.roberts@arm.com> 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@arm.com>
> ---
> 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.

> }
>
> 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
>
>