[PATCH v2 15/15] mm/memory: ignore writable bit in folio_pte_batch()

David Hildenbrand
2024-01-25T19:34:14Z

... 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) are expected to not
care.

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/memory.c | 30 ++++++++++++++++++++++++------
1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index b3f035fe54c8d..64442da3df6d4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -968,7 +968,7 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
pte = pte_mkclean(pte);
if (likely(flags & FPB_IGNORE_SOFT_DIRTY))
pte = pte_clear_soft_dirty(pte);
- return pte_mkold(pte);
+ return pte_wrprotect(pte_mkold(pte));
}

/*
@@ -976,21 +976,32 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
* 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).
+ * the accessed bit, writable bit, dirty bit (with FPB_IGNORE_DIRTY) and
+ * soft-dirty bit (with FPB_IGNORE_SOFT_DIRTY).
+ *
+ * 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, fpb_t flags)
+ pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
+ 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), flags);
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), flags);
+ pte = ptep_get(ptep);
+ if (any_writable)
+ writable = !!pte_write(pte);
+ pte = __pte_batch_clear_ignored(pte, flags);

if (!pte_same(pte, expected_pte))
break;
@@ -1003,6 +1014,9 @@ 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++;
}
@@ -1024,6 +1038,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
{
struct page *page;
struct folio *folio;
+ bool any_writable;
fpb_t flags = 0;
int err, nr;

@@ -1044,7 +1059,8 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
if (!vma_soft_dirty_enabled(src_vma))
flags |= FPB_IGNORE_SOFT_DIRTY;

- nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags);
+ nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags,
+ &any_writable);
folio_ref_add(folio, nr);
if (folio_test_anon(folio)) {
if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
@@ -1058,6 +1074,8 @@ 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