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

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Date:   Fri, 4 Jan 2019 10:25:12 -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 10:17 AM, Michal Hocko wrote:
> On Fri 04-01-19 10:01:40, Qian Cai wrote:
>> 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.
> 
> I am pretty sure we will hear about that when that happens. And act
> accordingly.
> 
>> 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.
> 
> Or simply make the page_owner initialization only touch the already
> initialized memory. Have you explored that option as well?

Yes, a proof-of-concept version is v1 where ends up with more ifdefs due to
dealing with all the low-level details,

https://lore.kernel.org/lkml/20181220060303.38686-1-cai@lca.pw/