linux-kernel - Re: [PATCH 7/7] mm/page_alloc: Replace local

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Date:   Tue, 21 Jun 2022 10:29:50 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal
 spinlock

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