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

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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





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