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Date:   Fri, 1 Feb 2019 14:51:39 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 11/22] mm, compaction: Use free lists to quickly locate a
 migration target
Message-ID: <20190201145139.GI9565@techsingularity.net>
References: <20190118175136.31341-1-mgorman@techsingularity.net>
 <20190118175136.31341-12-mgorman@techsingularity.net>
 <81e45dc0-c107-015b-e167-19d7ca4b6374@suse.cz>
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On Thu, Jan 31, 2019 at 03:52:10PM +0100, Vlastimil Babka wrote:
> > -/* Reorder the free list to reduce repeated future searches */
> > +/*
> > + * Used when scanning for a suitable migration target which scans freelists
> > + * in reverse. Reorders the list such as the unscanned pages are scanned
> > + * first on the next iteration of the free scanner
> > + */
> > +static void
> > +move_freelist_head(struct list_head *freelist, struct page *freepage)
> > +{
> > +	LIST_HEAD(sublist);
> > +
> > +	if (!list_is_last(freelist, &freepage->lru)) {
> 
> Shouldn't there be list_is_first() for symmetry?
> 

I don't think it would help. We're reverse traversing the list when this is
called. If it's the last entry, it's moving just one page before breaking
off the search and a shuffle has minimal impact. If it's the first page
then list_cut_before moves the entire list to sublist before splicing it
back so it's a pointless operation.

-- 
Mel Gorman
SUSE Labs