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Date:   Fri, 4 Jan 2019 16:32:45 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, Pavel.Tatashin@microsoft.com,
        mingo@kernel.org, mgorman@techsingularity.net,
        iamjoonsoo.kim@lge.com, tglx@linutronix.de, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm/page_owner: fix for deferred struct page init
Message-ID: <20190104153245.GV31793@dhcp22.suse.cz>
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On Fri 04-01-19 10:25:12, Qian Cai wrote:
> 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/

That is obviously not what I've had in mind. We have __init_single_page
which initializes a single struct page. Is there any way to hook
page_ext initialization there?

-- 
Michal Hocko
SUSE Labs