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Date:   Tue, 26 Feb 2019 15:23:52 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC
Message-ID: <20190226142352.GC10588@dhcp22.suse.cz>
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On Tue 26-02-19 09:16:30, Qian Cai wrote:
> 
> 
> On 2/26/19 7:35 AM, Michal Hocko wrote:
> > On Mon 25-02-19 14:17:10, Qian Cai wrote:
> >> When onlining memory pages, it calls kernel_unmap_linear_page(),
> >> However, it does not call kernel_map_linear_page() while offlining
> >> memory pages. As the result, it triggers a panic below while onlining on
> >> ppc64le as it checks if the pages are mapped before unmapping,
> >> Therefore, let it call kernel_map_linear_page() when setting all pages
> >> as reserved.
> > 
> > This really begs for much more explanation. All the pages should be
> > unmapped as they get freed AFAIR. So why do we need a special handing
> > here when this path only offlines free pages?
> > 
> 
> It sounds like this is exact the point to explain the imbalance. When offlining,
> every page has already been unmapped and marked reserved. When onlining, it
> tries to free those reserved pages via __online_page_free(). Since those pages
> are order 0, it goes free_unref_page() which in-turn call
> kernel_unmap_linear_page() again without been mapped first.

How is this any different from an initial page being freed to the
allocator during the boot?

-- 
Michal Hocko
SUSE Labs