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Subject: Re: [PATCH] mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC
To:     Michal Hocko <mhocko@kernel.org>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20190225191710.48131-1-cai@lca.pw>
 <20190226123521.GZ10588@dhcp22.suse.cz>
From:   Qian Cai <cai@lca.pw>
Message-ID: <4d4d3140-6d83-6d22-efdb-370351023aea@lca.pw>
Date:   Tue, 26 Feb 2019 09:16:30 -0500
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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.