linux-kernel - Re: [PATCH v3] mm/page_owner: fix for deferred struct page init

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Date:   Fri, 4 Jan 2019 10:01:40 -0500
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     akpm@...ux-foundation.org, Pavel.Tatashin@...rosoft.com,
        mingo@...nel.org, mgorman@...hsingularity.net,
        iamjoonsoo.kim@....com, tglx@...utronix.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/page_owner: fix for deferred struct page init

On 1/4/19 8:09 AM, Michal Hocko wrote:
>> Here is the number without DEFERRED_STRUCT_PAGE_INIT.
>>
>> == page_ext_init() after page_alloc_init_late() ==
>> Node 0, zone DMA: page owner found early allocated 0 pages
>> Node 0, zone DMA32: page owner found early allocated 7009 pages
>> Node 0, zone Normal: page owner found early allocated 85827 pages
>> Node 4, zone Normal: page owner found early allocated 75063 pages
>>
>> == page_ext_init() before kmemleak_init() ==
>> Node 0, zone DMA: page owner found early allocated 0 pages
>> Node 0, zone DMA32: page owner found early allocated 6654 pages
>> Node 0, zone Normal: page owner found early allocated 41907 pages
>> Node 4, zone Normal: page owner found early allocated 41356 pages
>>
>> So, it told us that it will miss tens of thousands of early page allocation call
>> sites.
> 
> This is an answer for the first part of the question (how much). The
> second is _do_we_care_?

Well, the purpose of this simple "ugly" ifdef is to avoid a regression for the
existing page_owner users with DEFERRED_STRUCT_PAGE_INIT deselected that would
start to miss tens of thousands early page allocation call sites.

The other option I can think of to not hurt your eyes is to rewrite the whole
page_ext_init(), init_page_owner(), init_debug_guardpage() to use all early
functions, so it can work in both with DEFERRED_STRUCT_PAGE_INIT=y and without.
However, I have a hard-time to convince myself it is a sensible thing to do.