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Date:   Mon, 4 Jan 2021 19:27:53 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
        Liang Li <liliangleo@didiglobal.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page
 allocation for __GFP_ZERO
Message-ID: <20210104192753.GD22407@casper.infradead.org>
References: <20201221163024.GA22532@open-light-1.localdomain>
 <0e8b6a2f-527d-7c77-efcf-04f21ef2a77c@intel.com>
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On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen wrote:
> On 12/21/20 8:30 AM, Liang Li wrote:
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -137,6 +137,9 @@ enum pageflags {
> >  #endif
> >  #ifdef CONFIG_64BIT
> >  	PG_arch_2,
> > +#endif
> > +#ifdef CONFIG_PREZERO_PAGE
> > +	PG_zero,
> >  #endif
> >  	__NR_PAGEFLAGS,
> 
> I don't think this is worth a generic page->flags bit.
> 
> There's a ton of space in 'struct page' for pages that are in the
> allocator.  Can't we use some of that space?

I was going to object to that too, but I think the entire approach is
flawed and needs to be thrown out.  It just nukes the caches in extremely
subtle and hard to measure ways, lowering overall system performance.