Re: [PATCH 1/3] mm: use_mm: fix for arches checking mm_users to optimize TLB flushes

[Rafael Aquini <aquini@redhat.com>]

On Sun, Feb 23, 2020 at 02:25:18PM -0500, Andrea Arcangeli wrote:
> alpha, ia64, mips, powerpc, sh, sparc are relying on a check on
> mm->mm_users to know if they can skip some remote TLB flushes for
> single threaded processes.
> 
> Most callers of use_mm() tend to invoke mmget_not_zero() or
> get_task_mm() before use_mm() to ensure the mm will remain alive in
> between use_mm() and unuse_mm().
> 
> Some callers however don't increase mm_users and they instead rely on
> serialization in __mmput() to ensure the mm will remain alive in
> between use_mm() and unuse_mm(). Not increasing mm_users during
> use_mm() is however unsafe for aforementioned arch TLB flushes
> optimizations. So either mmget()/mmput() should be added to the
> problematic callers of use_mm()/unuse_mm() or we can embed them in
> use_mm()/unuse_mm() which is more robust.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>  mm/mmu_context.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/mmu_context.c b/mm/mmu_context.c
> index 3e612ae748e9..ced0e1218c0f 100644
> --- a/mm/mmu_context.c
> +++ b/mm/mmu_context.c
> @@ -30,6 +30,7 @@ void use_mm(struct mm_struct *mm)
>  		mmgrab(mm);
>  		tsk->active_mm = mm;
>  	}
> +	mmget(mm);
>  	tsk->mm = mm;
>  	switch_mm(active_mm, mm, tsk);
>  	task_unlock(tsk);
> @@ -57,6 +58,7 @@ void unuse_mm(struct mm_struct *mm)
>  	task_lock(tsk);
>  	sync_mm_rss(mm);
>  	tsk->mm = NULL;
> +	mmput(mm);
>  	/* active_mm is still 'mm' */
>  	enter_lazy_tlb(mm, tsk);
>  	task_unlock(tsk);

Acked-by: Rafael Aquini <aquini@redhat.com>