Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal spinlock

Mel Gorman
2022-06-21T09:29:56Z

On Fri, Jun 17, 2022 at 11:39:03AM +0200, Nicolas Saenz Julienne wrote:
> Hi Mel,
>
> On Mon, 2022-06-13 at 13:56 +0100, Mel Gorman wrote:
> > @@ -3446,12 +3490,16 @@ void free_unref_page(struct page *page, unsigned int order)
> > migratetype = MIGRATE_MOVABLE;
> > }
> >
> > - local_lock_irqsave(&pagesets.lock, flags);
> > - freed_pcp = free_unref_page_commit(page, migratetype, order, false);
> > - local_unlock_irqrestore(&pagesets.lock, flags);
> > -
> > - if (unlikely(!freed_pcp))
> > + zone = page_zone(page);
> > + pcp_trylock_prepare(UP_flags);
>
> Now that you're calling the *_irqsave() family of function you can drop
> pcp_trylock_prepare/finish()
>
> For the record in UP:
>
> #define spin_trylock_irqsave(lock, flags) \
> ({ \
> local_irq_save(flags); \
> 1;
> })
>

The missing patch that is deferred for a later release uses spin_trylock
so unless that is never merged because there is an unfixable flaw in it,
I'd prefer to leave the preparation in place.

> > + pcp = pcpu_spin_trylock_irqsave(struct per_cpu_pages, lock, zone->per_cpu_pageset, flags);
> > + if (pcp) {
> > + free_unref_page_commit(pcp, zone, page, migratetype, order);
> > + pcp_spin_unlock_irqrestore(pcp, flags);
> > + } else {
> > free_one_page(page_zone(page), page, pfn, order, migratetype, FPI_NONE);
> > + }
> > + pcp_trylock_finish(UP_flags);
> > }
> >
> > /*
>
> As Vlastimil mentioned elsewhere, I also wonder if it makes sense to just
> bypass patch #5. Especially as its intent isn't true anymore:
>
> "As preparation for dealing with both of those problems, protect the lists
> with a spinlock. The IRQ-unsafe version of the lock is used because IRQs
> are already disabled by local_lock_irqsave. spin_trylock is used in
> preparation for a time when local_lock could be used instead of
> lock_lock_irqsave."
>

It's still true, the patch just isn't included as I wanted them to be
separated by time so a bisection that points to it is "obvious" instead
of pointing at the whole series as being a potential problem.

--
Mel Gorman
SUSE Labs