[PATCH 07/19] mm/fork: Accept huge pfnmap entries

Peter Xu
2024-08-09T16:09:33Z

Teach the fork code to properly copy pfnmaps for pmd/pud levels. Pud is
much easier, the write bit needs to be persisted though for writable and
shared pud mappings like PFNMAP ones, otherwise a follow up write in either
parent or child process will trigger a write fault.

Do the same for pmd level.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/huge_memory.c | 27 ++++++++++++++++++++++++---
1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6568586b21ab..015c9468eed5 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1375,6 +1375,22 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
pgtable_t pgtable = NULL;
int ret = -ENOMEM;

+ pmd = pmdp_get_lockless(src_pmd);
+ if (unlikely(pmd_special(pmd))) {
+ dst_ptl = pmd_lock(dst_mm, dst_pmd);
+ src_ptl = pmd_lockptr(src_mm, src_pmd);
+ spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
+ /*
+ * No need to recheck the pmd, it can't change with write
+ * mmap lock held here.
+ */
+ if (is_cow_mapping(src_vma->vm_flags) && pmd_write(pmd)) {
+ pmdp_set_wrprotect(src_mm, addr, src_pmd);
+ pmd = pmd_wrprotect(pmd);
+ }
+ goto set_pmd;
+ }
+
/* Skip if can be re-fill on fault */
if (!vma_is_anonymous(dst_vma))
return 0;
@@ -1456,7 +1472,9 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
pmdp_set_wrprotect(src_mm, addr, src_pmd);
if (!userfaultfd_wp(dst_vma))
pmd = pmd_clear_uffd_wp(pmd);
- pmd = pmd_mkold(pmd_wrprotect(pmd));
+ pmd = pmd_wrprotect(pmd);
+set_pmd:
+ pmd = pmd_mkold(pmd);
set_pmd_at(dst_mm, addr, dst_pmd, pmd);

ret = 0;
@@ -1502,8 +1520,11 @@ int copy_huge_pud(struct mm_struct *dst_mm, struct mm_struct *src_mm,
* TODO: once we support anonymous pages, use
* folio_try_dup_anon_rmap_*() and split if duplicating fails.
*/
- pudp_set_wrprotect(src_mm, addr, src_pud);
- pud = pud_mkold(pud_wrprotect(pud));
+ if (is_cow_mapping(vma->vm_flags) && pud_write(pud)) {
+ pudp_set_wrprotect(src_mm, addr, src_pud);
+ pud = pud_wrprotect(pud);
+ }
+ pud = pud_mkold(pud);
set_pud_at(dst_mm, addr, dst_pud, pud);

ret = 0;
--
2.45.0