linux-kernel - Re: [PATCH RFC 4/4] mm/page_alloc: place pages to tail in __free_pages

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Message-ID: <a18327c0-b86a-df00-e984-27c26468caf7@redhat.com>
Date:   Mon, 28 Sep 2020 10:36:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-acpi@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Mike Rapoport <rppt@...nel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>
Subject: Re: [PATCH RFC 4/4] mm/page_alloc: place pages to tail in
 __free_pages_core()

On 28.09.20 09:58, Oscar Salvador wrote:
> 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?

Hi Oscar!

Old code:

set_page_refcounted(): sets the refcount to 1.
__free_pages()
  -> put_page_testzero(): sets it to 0
  -> free_the_page()->__free_pages_ok()

New code:

set_page_refcounted(): sets the refcount to 1.
page_ref_dec(page): sets it to 0
__free_pages_ok():


We could skip the set_page_refcounted() + page_ref_dec(page) and lose a
couple of sanity checks but we could simply use a
VM_BUG_ON_PAGE(page_ref_count(page), page), which is what we really care
about when onlining memory.

-- 
Thanks,

David / dhildenb