Re: [PATCH 1/2] mm: use_mm: fix for arches checking mm_users to optimize TLB flushes

[Michal Hocko]

On Mon 03-02-20 15:17:44, 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.

I would prefer we do not do that because then the real owner of the mm
cannot really tear down the address space and the life time of it is
bound to a kernel thread doing the use_mm. This is undesirable I would
really prefer if the existing few users would use mmget only when they
really need to access mm.

> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
>  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);
> 
> 
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-- 
Michal Hocko
SUSE Labs