Re: [PATCH v3 30/40] mm/madvise: update is_valid_guard_vma() to use vma_can_merge()

We currently disallow the installation of lightweight guard regions in VMAs
whose flags intersect VMA_SPECIAL_FLAGS or VMA_HUGETLB_BIT, or
VMA_LOCKED_BIT unless allow_locked is set.

hugetlb VMAs set VMA_DONTEXPAND_BIT so this was already redundant,
VMA_SPECIAL_FLAGS already sufficed.

However, now that VMA_IO_BIT is only set if VMA_PFNMAP or VMA_MIXEDMAP_BIT
is set, this check collapses to being the equivalent of
!vma_can_merge().

Update is_valid_guard_vma() to reflect this.

No functional change intended.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/madvise.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 467601a8525b..0922d5f07a12 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -1221,19 +1221,25 @@ static long madvise_remove(struct madvise_behavior *madv_behavior)
 	return error;
 }
 
-static bool is_valid_guard_vma(struct vm_area_struct *vma, bool allow_locked)
+static bool is_valid_guard_vma(const struct vm_area_struct *vma,
+			       bool allow_locked)
 {
-	vm_flags_t disallowed = VM_SPECIAL | VM_HUGETLB;
-
 	/*
-	 * A user could lock after setting a guard range but that's fine, as
+	 * A user could lock after setting a guard range but that's fine as
 	 * they'd not be able to fault in. The issue arises when we try to zap
 	 * existing locked VMAs. We don't want to do that.
 	 */
-	if (!allow_locked)
-		disallowed |= VM_LOCKED;
+	if (!allow_locked && vma_test(vma, VMA_LOCKED_BIT))
+		return false;
+	/*
+	 * Guard regions require a VMA whose page tables are managed solely by
+	 * the core, which is also what merging requires, so disallow any flags
+	 * that would prevent a merge.
+	 */
+	if (!vma_can_merge(vma))
+		return false;