Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Michal Hocko
2020-08-11T08:19:21Z

On Mon 10-08-20 21:25:26, Michal Hocko wrote:
> On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
[...]
> > The problem that i see is we can not use the page allocator from atomic
> > contexts, what is our case:
> >
> > <snip>
> > local_irq_save(flags) or preempt_disable() or raw_spinlock();
> > __get_free_page(GFP_ATOMIC);
> > <snip>
> >
> > So if we can convert the page allocator to raw_* lock it will be appreciated,
> > at least from our side, IMHO, not from RT one. But as i stated above we need
> > to sort raised questions out if converting is done.
> >
> > What is your view?
>
> To me it would make more sense to support atomic allocations also for
> the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really
> work for atomic context in RT sounds subtle and wrong.

I was thinking about this some more. I still think the above would be a
reasonable goal we should try to achieve. If for not other then for
future maintainability (especially after the RT patchset is merged).
I have tried to search for any known problems/attempts to make
zone->lock raw but couldn't find anything. Maybe somebody more involved
in RT world have something to say about that.

Anyway, if the zone->lock is not a good fit for raw_spin_lock then the
only way I can see forward is to detect real (RT) atomic contexts and
bail out early before taking the lock in the allocator for NOWAIT/ATOMIC
requests.
--
Michal Hocko
SUSE Labs