[PATCH NOTFORMERGE 4/5] mm/madvise: employ mmget_still_valid for write lock

[Oleksandr Natalenko]

Do the very same trick as we already do since 04f5866e41fb. KSM hints
will require locking mmap_sem for write since they modify vm_flags, so
for remote KSM hinting this additional check is needed.

Signed-off-by: Oleksandr Natalenko <oleksa...@redhat.com>
---
 mm/madvise.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index 9755340da157..84f899b1b6da 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1049,6 +1049,8 @@ static int madvise_common(struct task_struct *task, 
struct mm_struct *mm,
        if (write) {
                if (down_write_killable(&mm->mmap_sem))
                        return -EINTR;
+               if (current->mm != mm && !mmget_still_valid(mm))
+                       goto skip_mm;
        } else {
                down_read(&mm->mmap_sem);
        }
@@ -1099,6 +1101,7 @@ static int madvise_common(struct task_struct *task, 
struct mm_struct *mm,
        }
 out:
        blk_finish_plug(&plug);
+skip_mm:
        if (write)
                up_write(&mm->mmap_sem);
        else
-- 
2.22.0