Re: [PATCH v6 6/7] mm/madvise: employ mmget_still_valid for write lock

On Sun, Mar 1, 2020 at 11:33 PM Oleksandr Natalenko
[off-list ref] wrote:

Hello.

On Fri, Feb 28, 2020 at 03:19:55PM -0800, Suren Baghdasaryan wrote:

quoted

On Tue, Feb 18, 2020 at 5:44 PM Minchan Kim [off-list ref] wrote:

quoted

From: Oleksandr Natalenko <redacted>

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 <redacted>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 mm/madvise.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index f6d9b9e66243..c55a18fe71f9 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -1118,6 +1118,8 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
        if (write) {
                if (down_write_killable(&mm->mmap_sem))
                        return -EINTR;
+               if (current->mm != mm && !mmget_still_valid(mm))

mmget_still_valid() seems pretty light-weight, so why not just use
that without checking that the mm belongs to the current process
first?

I'd keep the checks separate to a) do not functionally change current->mm
== mm case; b) clearly separate the intention to call
mmget_still_valid() only for remote access (using mmget_still_valid()
for current->mm == mm does not make any sense here, IMO, since there's
no possibility of expecting a core dump at this point); c) ease the job for
reviewer once mmget_still_valid() is scheduled to be removed (I hope it
eventually goes away indeed).

Makes sense. Thanks!

quoted

quoted

+                       goto skip_mm;
        } else {
                down_read(&mm->mmap_sem);
        }

@@ -1169,6 +1171,7 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
        }
 out:
        blk_finish_plug(&plug);
+skip_mm:
        if (write)
                up_write(&mm->mmap_sem);
        else
--

2.25.0.265.gbab2e86ba0-goog

--
  Best regards,
    Oleksandr Natalenko (post-factum)
    Principal Software Maintenance Engineer

Reviewed-by: Suren Baghdasaryan <surenb@google.com>