linux-kernel - Re: [v5 1/2] mm: disable interrupts while initializing deferred pages

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Message-ID: <20180313160430.hbjnyiazadt3jwa6@xakep.localdomain>
Date:   Tue, 13 Mar 2018 12:04:30 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        m.mizuma@...fujitsu.com, mhocko@...e.com, catalin.marinas@....com,
        takahiro.akashi@...aro.org, gi-oh.kim@...fitbricks.com,
        heiko.carstens@...ibm.com, baiyaowei@...s.chinamobile.com,
        richard.weiyang@...il.com, paul.burton@...s.com,
        miles.chen@...iatek.com, vbabka@...e.cz, mgorman@...e.de,
        hannes@...xchg.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [v5 1/2] mm: disable interrupts while initializing deferred pages

Hi Andrew,

> > +/* Disable interrupts and save previous IRQ state in flags before locking */
> > +static inline
> > +void pgdat_resize_lock_irq(struct pglist_data *pgdat, unsigned long *flags)
> > +{
> > +	unsigned long tmp_flags;
> > +
> > +	local_irq_save(*flags);
> > +	local_irq_disable();
> > +	pgdat_resize_lock(pgdat, &tmp_flags);
> > +}
> 
> As far as I can tell, this ugly-looking thing is identical to
> pgdat_resize_lock().

I will get rid of it, and use pgdat_resize_lock(). My confusion was that I
thought that local_irq_save() only saves the IRQ flags does not disable
them.

> 
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1506,7 +1506,6 @@ static void __init deferred_free_pages(int nid, int zid, unsigned long pfn,
> >  		} else if (!(pfn & nr_pgmask)) {
> >  			deferred_free_range(pfn - nr_free, nr_free);
> >  			nr_free = 1;
> > -			cond_resched();
> >  		} else {
> >  			nr_free++;
> 
> And how can we simply remove these cond_resched()s?  I assume this is
> being done because interrupts are now disabled?  But those were there
> for a reason, weren't they?

We must remove cond_resched() because we can't sleep anymore. They were
added to fight NMI timeouts, so I will replace them with
touch_nmi_watchdog() in a follow-up fix.

Thank you for your review,
Pavel