linux-kernel - Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs

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Date:   Fri, 28 May 2021 11:09:18 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     David Hildenbrand <david@...hat.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, "Tang, Feng" <feng.tang@...el.com>
Subject: Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and
 active CPUs

On Fri, May 28, 2021 at 11:52:53AM +0200, David Hildenbrand wrote:
> > > "Disable pcplists so that page isolation cannot race with freeing
> > >   in a way that pages from isolated pageblock are left on pcplists."
> > > 
> > > Guess we'd then want to move the draining before start_isolate_page_range()
> > > in alloc_contig_range().
> > > 
> > 
> > Or instead of draining, validate the PFN range in alloc_contig_range
> > is within the same zone and if so, call zone_pcp_disable() before
> > start_isolate_page_range and enable after __alloc_contig_migrate_range.
> > 
> 
> We require the caller to only pass a range within a single zone, so that
> should be fine.
> 
> The only ugly thing about zone_pcp_disable() is
> mutex_lock(&pcp_batch_high_lock) which would serialize all
> alloc_contig_range() and even with offline_pages().
> 

True so it would have to be accessed if that is bad or not. If racing
against offline_pages, memory is potentially being offlined in the
target zone which may cause allocation failure. If racing with other
alloc_contig_range calls, the two callers are potentially racing to
isolate and allocate the same range. The arguement could be made that
alloc_contig_ranges should be serialised within one zone to improve the
allocation success rate at the potential cost of allocation latency.

-- 
Mel Gorman
SUSE Labs