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

[Oleksandr Natalenko]

Hello.

On Fri, Feb 28, 2020 at 03:19:55PM -0800, Suren Baghdasaryan wrote:
> On Tue, Feb 18, 2020 at 5:44 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
> >
> > From: Oleksandr Natalenko <oleksandr@xxxxxxxxxx>
> >
> > 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 <oleksandr@xxxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> >  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).

> 
> > +                       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