linux-kernel - Re: [PATCH 2/6] mm/page_alloc: Convert per-cpu list protection to local

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Message-ID: <20210331204210.GB3697@techsingularity.net>
Date:   Wed, 31 Mar 2021 21:42:10 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux-RT-Users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH 2/6] mm/page_alloc: Convert per-cpu list protection to
 local_lock

On Wed, Mar 31, 2021 at 07:42:42PM +0200, Thomas Gleixner wrote:
> On Wed, Mar 31 2021 at 12:01, Mel Gorman wrote:
> > On Wed, Mar 31, 2021 at 11:55:56AM +0200, Thomas Gleixner wrote:
> > @@ -887,13 +887,11 @@ void cpu_vm_stats_fold(int cpu)
> >  
> >  		pzstats = per_cpu_ptr(zone->per_cpu_zonestats, cpu);
> >  
> > -		preempt_disable();
> >  		for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++)
> >  			if (pzstats->vm_stat_diff[i]) {
> >  				int v;
> >  
> > -				v = pzstats->vm_stat_diff[i];
> > -				pzstats->vm_stat_diff[i] = 0;
> > +				v = this_cpu_xchg(pzstats->vm_stat_diff[i], 0);
> 
> Confused. pzstats is not a percpu pointer. zone->per_cpu_zonestats is.
> 
> But @cpu is not necessarily the current CPU.
> 

I was drinking drain cleaner instead of coffee. The code was also broken
to begin with.

drain_pages() is draining pagesets of a local or dead CPU. For a local
CPU, disabling IRQs prevent an IRQ arriving during the drain, trying to
allocate a page and potentially corrupt the local pageset -- ok.

zone_pcp_reset is accessing a remote CPUs pageset, freeing the percpu
pointer and resetting it to boot_pageset. zone_pcp_reset calling
local_irq_save() does not offer any special protection against
drain_pages because there are two separate IRQs involved.

This particular patch may have no reason to touch zone_pcp_reset,
cpu_vm_stats_fold or drain_zonestat at all but I need to think about it
more tomorrow.

-- 
Mel Gorman
SUSE Labs