[PATCH v1 11/11] mm/memory: ignore writable bit in folio_pte_batch()

David Hildenbrand
2024-01-22T19:43:54Z

... and conditionally return to the caller if any pte except the first one
is writable. fork() has to make sure to properly write-protect in case any
PTE is writable. Other users (e.g., page unmaping) won't care.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/memory.c | 26 +++++++++++++++++++++-----
1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 341b2be845b6e..a26fd0669016b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -955,7 +955,7 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,

static inline pte_t __pte_batch_clear_ignored(pte_t pte)
{
- return pte_clear_soft_dirty(pte_mkclean(pte_mkold(pte)));
+ return pte_wrprotect(pte_clear_soft_dirty(pte_mkclean(pte_mkold(pte))));
}

/*
@@ -963,20 +963,29 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte)
* 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 and soft-dirty bit.
+ * the accessed bit, dirty bit, soft-dirty bit and writable bit.
+ . If "any_writable" is set, it will indicate if any other PTE besides the
+ * first (given) PTE is writable.
*/
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, bool *any_writable)
{
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_batch_clear_ignored(pte_next_pfn(pte));
pte_t *ptep = start_ptep + 1;
+ bool writable;
+
+ if (any_writable)
+ *any_writable = false;

VM_WARN_ON_FOLIO(!pte_present(pte), folio);

while (ptep != end_ptep) {
- pte = __pte_batch_clear_ignored(ptep_get(ptep));
+ pte = ptep_get(ptep);
+ if (any_writable)
+ writable = !!pte_write(pte);
+ pte = __pte_batch_clear_ignored(pte);

if (!pte_same(pte, expected_pte))
break;
@@ -989,6 +998,9 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
if (pte_pfn(pte) == folio_end_pfn)
break;

+ if (any_writable)
+ *any_writable |= writable;
+
expected_pte = pte_next_pfn(expected_pte);
ptep++;
}
@@ -1010,6 +1022,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
{
struct page *page;
struct folio *folio;
+ bool any_writable;
int err, nr;

page = vm_normal_page(src_vma, addr, pte);
@@ -1024,7 +1037,8 @@ 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);
+ nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr,
+ &any_writable);
if (folio_test_anon(folio)) {
folio_ref_add(folio, nr);
if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
@@ -1039,6 +1053,8 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
folio_dup_file_rmap_ptes(folio, page, nr);
rss[mm_counter_file(page)] += nr;
}
+ if (any_writable)
+ pte = pte_mkwrite(pte, src_vma);
__copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte,
addr, nr);
return nr;
--
2.43.0