On 09.08.24 18:08, Peter Xu wrote:
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@xxxxxxxxxx>
---
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;
+ }
+
I strongly assume we should be using using vm_normal_page_pmd() instead
of pmd_page() further below. pmd_special() should be mostly limited to
GUP-fast and vm_normal_page_pmd().
Again, we should be doing this similar to how we handle PTEs.
I'm a bit confused about the "unlikely(!pmd_trans_huge(pmd)" check,
below: what else should we have here if it's not a migration entry but a
present entry?
Likely this function needs a bit of rework.
--
Cheers,
David / dhildenb
