Re: [PATCH] mm/mempolicy: fix a race between offset_il_node and mpol_rebind_task

[Muchun Song <songmuchun@bytedance.com>]

On Sat, Aug 14, 2021 at 12:58 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Sat, Aug 14, 2021 at 12:40:53AM +0800, yanghui wrote:
> > +++ b/mm/mempolicy.c
> > @@ -193,7 +193,7 @@ static int mpol_new_interleave(struct mempolicy *pol, const nodemask_t *nodes)
> >  {
> >       if (nodes_empty(*nodes))
> >               return -EINVAL;
> > -     pol->nodes = *nodes;
> > +     WRITE_ONCE(pol->nodes, *nodes);
>
> typedef struct { DECLARE_BITMAP(bits, MAX_NUMNODES); } nodemask_t;
>
> If MAX_NUMNODES is large enough, is WRITE_ONCE going to work?  It could
> be 128 bits, and few architectures have an atomic 128-bit store
> instruction.
>

Hi Matthew,

In my memory, the WRITE_ONCE will become memcpy if
the size is greater than sizeof(long). But I found that it didn't
support the size over sizeof(long) nowadays. So you are
right. It doesn't work when MAX_NUMNODES is large enough.

If we don't use WRITE_ONCE/READ_ONCE here, it seems
like no problem. At least we can sure offset_il_node returns
a valid node id. What is your opinion about this fix?

Thanks,
Muchun