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Date:   Mon, 28 Sep 2020 09:58:24 +0200
From:   Oscar Salvador <osalvador@suse.de>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-hyperv@vger.kernel.org, xen-devel@lists.xenproject.org,
        linux-acpi@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Wei Yang <richard.weiyang@linux.alibaba.com>,
        Mike Rapoport <rppt@kernel.org>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>
Subject: Re: [PATCH RFC 4/4] mm/page_alloc: place pages to tail in
 __free_pages_core()
Message-ID: <20200928075820.GA4082@linux>
References: <20200916183411.64756-1-david@redhat.com>
 <20200916183411.64756-5-david@redhat.com>
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On Wed, Sep 16, 2020 at 08:34:11PM +0200, David Hildenbrand wrote:
> @@ -1523,7 +1524,13 @@ void __free_pages_core(struct page *page, unsigned int order)
>  
>  	atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
>  	set_page_refcounted(page);
> -	__free_pages(page, order);
> +
> +	/*
> +	 * Bypass PCP and place fresh pages right to the tail, primarily
> +	 * relevant for memory onlining.
> +	 */
> +	page_ref_dec(page);
> +	__free_pages_ok(page, order, FOP_TO_TAIL);

Sorry, I must be missing something obvious here, but I am a bit confused here.
I get the part of placing them at the tail so rmqueue_bulk() won't
find them, but I do not get why we decrement page's refcount.
IIUC, its refcount will be 0, but why do we want to do that?

Another thing a bit unrelated... we mess three times with page's refcount
(two before this patch).
Why do we have this dance in place?

Thanks

-- 
Oscar Salvador
SUSE L3