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

Nicolas Saenz Julienne
2022-06-21T09:31:51Z

On Tue, 2022-06-21 at 10:29 +0100, Mel Gorman wrote:
> 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.

Understood, I jumped straight into the code and missed your comment in the
cover letter.

Thanks!

--
Nicolás Sáenz