Make it clear what we're blocking in MADV_DOFORK. Previously we simply
disallowed VM_SPECIAL i.e. mappings not managed by core mm, fixed mappings
and VMA_IO_BIT.
Now the invariant is established that mm-managed mappings may not set
VMA_IO_BIT, the VMA_IO_BIT check is redundant.
The rest is equivalent to testing for a fixed or non-mm-managed mapping,
i.e. a mapping which is not mm-backed, or hugetlb, which is specifically
excluded.
This was established by commit 0b2758f48f22 ("Require (reasonably) normal
mappings for MADV_DOFORK") containing my hands-down favourite call out of
all time.
Express the same thing using the predicates - only mm-backed mappings may
have their copy-on-fork behaviour changed, retaining the eternal exception
to all rules, hugetlb.
No functional change intended.
Acked-by: Zi Yan <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>
---
mm/madvise.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/madvise.c b/mm/madvise.c
index 7e14b3fd92f6..e050a21e4db1 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1565,7 +1565,7 @@ static int madvise_vma_behavior(struct madvise_behavior
*madv_behavior)
new_flags |= VM_DONTCOPY;
break;
case MADV_DOFORK:
- if (new_flags & VM_SPECIAL)
+ if (!vma_is_mm_backed(vma) || vma_is_hugetlb(vma))
return -EINVAL;
new_flags &= ~VM_DONTCOPY;
break;
--
2.55.0
