Re: [PATCH v2 1/2] mm: Abstract THP allocation

On 9/5/24 13:50, Kirill A. Shutemov wrote:

On Wed, Sep 04, 2024 at 03:39:22PM +0530, Dev Jain wrote:

quoted

In preparation for the second patch, abstract away the THP allocation
logic present in the create_huge_pmd() path, which corresponds to the
faulting case when no page is present.

There should be no functional change as a result of applying
this patch.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
  mm/huge_memory.c | 110 +++++++++++++++++++++++++++++------------------
  1 file changed, 67 insertions(+), 43 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 67c86a5d64a6..58125fbcc532 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -943,47 +943,89 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
  }
  EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
  
-static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
-			struct page *page, gfp_t gfp)
+static vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
+				  unsigned long haddr, struct folio **foliop,
+				  unsigned long addr)

foliop is awkward.

Why not return folio? NULL would indicate to the caller to fallback.

I took inspiration from other call sites like mfill_atomic_pte_copy() which
have a double pointer to the folio. If we do as you say, then in thp_fault_alloc(),
we will have to do all the fallback stat computation, and return the folio, then
check in the caller whether that is NULL, then set ret = VM_FAULT_FALLBACK, then
goto out/release. Basically, stat computation for fallback, and the actual setting
of return value to fallback would get separated. Currently, I can simultaneously
set the folio pointer and the return value.

quoted

  {
-	struct vm_area_struct *vma = vmf->vma;
-	struct folio *folio = page_folio(page);
-	pgtable_t pgtable;
-	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
-	vm_fault_t ret = 0;
+	struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
  
-	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
+	*foliop = folio;
+	if (unlikely(!folio)) {
+		count_vm_event(THP_FAULT_FALLBACK);
+		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
+		return VM_FAULT_FALLBACK;
+	}
  
+	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
  	if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
  		folio_put(folio);
  		count_vm_event(THP_FAULT_FALLBACK);
  		count_vm_event(THP_FAULT_FALLBACK_CHARGE);
-		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
-		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
+		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
+		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
  		return VM_FAULT_FALLBACK;
  	}
  	folio_throttle_swaprate(folio, gfp);
  
-	pgtable = pte_alloc_one(vma->vm_mm);
-	if (unlikely(!pgtable)) {
-		ret = VM_FAULT_OOM;
-		goto release;
-	}
-
-	folio_zero_user(folio, vmf->address);
+	folio_zero_user(folio, addr);
  	/*
  	 * The memory barrier inside __folio_mark_uptodate makes sure that
  	 * folio_zero_user writes become visible before the set_pmd_at()
  	 * write.
  	 */
  	__folio_mark_uptodate(folio);
+	return 0;
+}
+
+static void __thp_fault_success_stats(struct vm_area_struct *vma, int order)
+{
+	count_vm_event(THP_FAULT_ALLOC);
+	count_mthp_stat(order, MTHP_STAT_ANON_FAULT_ALLOC);
+	count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
+}
+
+static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
+			struct vm_area_struct *vma, unsigned long haddr,
+			pgtable_t pgtable)
+{
+	pmd_t entry;
+
+	entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
+	entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
+	folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
+	folio_add_lru_vma(folio, vma);
+	pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
+	set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
+	update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
+	add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
+	mm_inc_nr_ptes(vma->vm_mm);
+}
+
+static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	struct folio *folio = NULL;
+	pgtable_t pgtable;
+	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+	vm_fault_t ret = 0;
+	gfp_t gfp = vma_thp_gfp_mask(vma);
+
+	pgtable = pte_alloc_one(vma->vm_mm);
+	if (unlikely(!pgtable)) {
+		ret = VM_FAULT_OOM;
+		goto release;
+	}
+
+	ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
+			      vmf->address);
+	if (ret)
+		goto release;

THP page allocation has higher probability to fail than pgtable allocation. It
is better to allocate it first, before pgtable and do less work on error
path.

Thanks, makes sense.