Re: [PATCH v1 10/11] mm/memory: ignore dirty/accessed/soft-dirty bits in folio_pte_batch()

David Hildenbrand
2024-01-23T14:13:41Z

>> Although now I'm wondering if there is a race here... What happens if a page in
>> the parent becomes dirty after you have checked it but before you write protect
>> it? Isn't that already a problem with the current non-batched version? Why do we
>> even to preserve dirty in the child for private mappings?
>
> I suspect, because the parent could zap the anon folio. If the folio is
> clean, but the PTE dirty, I suspect that we could lose data of the child
> if we were to evict that clean folio (swapout).
>
> So I assume we simply copy the dirty PTE bit, so the system knows that
> that folio is actually dirty, because one PTE is dirty.

Oh, and regarding your race concern: it's undefined which page state
would see if some write is racing with fork, so it also doesn't matter
if we would copy the PTE dirty bit or not, if it gets set in a racy fashion.

I'll not experiment with:

From 14e83ff2a422a96ce5701f9c8454a49f9ed947e3 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Sat, 30 Dec 2023 12:54:35 +0100
Subject: [PATCH] mm/memory: ignore dirty/accessed/soft-dirty bits in
folio_pte_batch()

Let's always ignore the accessed/young bit: we'll always mark the PTE
as old in our child process during fork, and upcoming users will
similarly not care.

Ignore the dirty bit only if we don't want to duplicate the dirty bit
into the child process during fork. Maybe, we could just set all PTEs
in the child dirty if any PTE is dirty. For now, let's keep the behavior
unchanged.

Ignore the soft-dirty bit only if the bit doesn't have any meaning in
the src vma.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/memory.c | 34 ++++++++++++++++++++++++++++++----
1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7690994929d26..9aba1b0e871ca 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -953,24 +953,44 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
}

+/* Flags for folio_pte_batch(). */
+typedef int __bitwise fpb_t;
+
+/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
+#define FPB_IGNORE_DIRTY ((__force fpb_t)BIT(0))
+
+/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
+#define FPB_IGNORE_SOFT_DIRTY ((__force fpb_t)BIT(1))
+
+static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
+{
+ if (flags & FPB_IGNORE_DIRTY)
+ pte = pte_mkclean(pte);
+ if (likely(flags & FPB_IGNORE_SOFT_DIRTY))
+ pte = pte_clear_soft_dirty(pte);
+ return pte_mkold(pte);
+}
+
/*
* Detect a PTE batch: consecutive (present) PTEs that map consecutive
* pages of the same folio.
*
* All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN.
+ * the accessed bit, dirty bit (with FPB_IGNORE_DIRTY) and soft-dirty bit
+ * (with FPB_IGNORE_SOFT_DIRTY).
*/
static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
- pte_t *start_ptep, pte_t pte, int max_nr)
+ pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags)
{
unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
const pte_t *end_ptep = start_ptep + max_nr;
- pte_t expected_pte = pte_next_pfn(pte);
+ pte_t expected_pte = __pte_batch_clear_ignored(pte_next_pfn(pte), flags);
pte_t *ptep = start_ptep + 1;

VM_WARN_ON_FOLIO(!pte_present(pte), folio);

while (ptep != end_ptep) {
- pte = ptep_get(ptep);
+ pte = __pte_batch_clear_ignored(ptep_get(ptep), flags);

if (!pte_same(pte, expected_pte))
break;
@@ -1004,6 +1024,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
{
struct page *page;
struct folio *folio;
+ fpb_t flags = 0;
int err, nr;

page = vm_normal_page(src_vma, addr, pte);
@@ -1018,7 +1039,12 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
* by keeping the batching logic separate.
*/
if (unlikely(!*prealloc && folio_test_large(folio) && max_nr != 1)) {
- nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr);
+ if (src_vma->vm_flags & VM_SHARED)
+ flags |= FPB_IGNORE_DIRTY;
+ if (!vma_soft_dirty_enabled(src_vma))
+ flags |= FPB_IGNORE_SOFT_DIRTY;
+
+ nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags);
folio_ref_add(folio, nr);
if (folio_test_anon(folio)) {
if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
--
2.43.0

--
Cheers,

David / dhildenb