[PATCH 3/4] mm/madvise: preserve VMA_SOFTDIRTY_BIT when changing anonymous names

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

When changing the anonymous VMA name, madvise_update_vma() calls
vma_modify_name() to possibly merge with adjacent VMAs. We don't call
vma_modify_flags() and consequently don't obtain the new VMA flags.

Later in the function we update the VMA flags to the stale new_vma_flags
we obtained before. This can result in us losing the VMA_SOFTDIRTY_BIT
flag set on a VMA we just merged with.

There is no need to update the VMA flags at all, so just exit early. This
skips calling khugepaged_enter_vma(), but since the flags didn't change
this is fine.

Fixes: 9a10064f5625 ("mm: add a field to store names for private anonymous memory")
Cc: <stable@vger.kernel.org>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
[ ljs - Minor edit of commit message, rebased ]
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/madvise.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index b2e00efcaa54..e9fa4f71efea 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -177,6 +177,9 @@ static int madvise_update_vma(vm_flags_t new_flags,
 
 	/* vm_flags is protected by the mmap_lock held in write mode. */
 	vma_start_write(vma);
+	if (set_new_anon_name)
+		return replace_anon_vma_name(vma, anon_name);
+
 	vma_overwrite_flags(vma, new_vma_flags);
 	/*
 	 * If the vma become good for khugepaged to scan,
@@ -186,9 +189,6 @@ static int madvise_update_vma(vm_flags_t new_flags,
 	if (vma_flags_test(&new_vma_flags, VMA_HUGEPAGE_BIT))
 		khugepaged_enter_vma(vma, vma_flags_to_legacy(new_vma_flags));
 
-	if (set_new_anon_name)
-		return replace_anon_vma_name(vma, anon_name);
-
 	return 0;
 }
 

-- 
2.56.0