+ mm-huge_memory-check-pmd_special-only-after-pmd_present.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/huge_memory: check pmd_special() only after pmd_present()
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-huge_memory-check-pmd_special-only-after-pmd_present.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-huge_memory-check-pmd_special-only-after-pmd_present.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: mm/huge_memory: check pmd_special() only after pmd_present()
Date: Thu, 26 Sep 2024 17:42:34 +0200

We should only check for pmd_special() after we made sure that we have a
present PMD.  For example, if we have a migration PMD, pmd_special() might
indicate that we have a special PMD although we really don't.

This fixes confusing migration entries as PFN mappings, and not doing what
we are supposed to do in the "is_swap_pmd()" case further down in the
function -- including messing up COW, page table handling and accounting.

Link: https://lkml.kernel.org/r/20240926154234.2247217-1-david@redhat.com
Fixes: bc02afbd4d73 ("mm/fork: accept huge pfnmap entries")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: syzbot+bf2c35fa302ebe3c7471@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/66f15c8d.050a0220.c23dd.000f.GAE@google.com/
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/huge_memory.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/huge_memory.c~mm-huge_memory-check-pmd_special-only-after-pmd_present
+++ a/mm/huge_memory.c
@@ -1586,7 +1586,7 @@ int copy_huge_pmd(struct mm_struct *dst_
 	int ret = -ENOMEM;
 
 	pmd = pmdp_get_lockless(src_pmd);
-	if (unlikely(pmd_special(pmd))) {
+	if (unlikely(pmd_present(pmd) && 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);
_

Patches currently in -mm which might be from david@redhat.com are

mm-huge_memory-check-pmd_special-only-after-pmd_present.patch
selftests-mm-hugetlb_fault_after_madv-use-default-hguetlb-page-size.patch
selftests-mm-hugetlb_fault_after_madv-improve-test-output.patch