Re: [PATCH v7 6/7] mm/madvise: employ mmget_still_valid for write lock

[Vlastimil Babka]

On 3/2/20 8:36 PM, Minchan Kim 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.
> 
> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> 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 e794367f681e..e77c6c1fad34 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))
> +			goto skip_mm;

This will return 0, is that correct? Shoudln't there be a similar error e.g. as
when finding the task by pid fails (-ESRCH ?), because IIUC the task here is
going away and dumping the core?

>  	} 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
>