linux-kernel - Re: [PATCH] mm/hotplug: fix an imbalance with DEBUG

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Message-ID: <20190226123521.GZ10588@dhcp22.suse.cz>
Date:   Tue, 26 Feb 2019 13:35:21 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC

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?

-- 
Michal Hocko
SUSE Labs