[RFC v2 PATCH 06/17] mm: Allocate large folios for anonymous memory

Ryan Roberts
2023-04-14T13:05:02Z

Add the machinery to determine what order of folio to allocate within
do_anonymous_page() and deal with racing faults to the same region.

For now, the maximum order is set to 4. This should probably be set
per-vma based on factors, and adjusted dynamically.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
mm/memory.c | 154 ++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 138 insertions(+), 16 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index d7e34a8c46aa..f92a28064596 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3030,6 +3030,90 @@ static inline int max_anon_folio_order(struct vm_area_struct *vma)
return ANON_FOLIO_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(*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
*
@@ -4058,6 +4142,9 @@ 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 = max_anon_folio_order(vma);
+ int pgcount = BIT(order);

/* File mapping without ->vm_ops ? */
if (vma->vm_flags & VM_SHARED)
@@ -4099,24 +4186,42 @@ 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;
+ 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;
- cgroup_throttle_swaprate(&folio->page, GFP_KERNEL);
+ 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);

@@ -4125,11 +4230,26 @@ 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 (!pte_none(*vmf->pte)) {
- 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 (unlikely(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 (order == 0 || !pte_none(*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;
}

ret = check_stable_address_space(vma->vm_mm);
@@ -4143,14 +4263,16 @@ 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:
- 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