linux-kernel - Re: [PATCH RFC v1] mm: is_mem_section

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Message-ID: <20200117113353.GT19428@dhcp22.suse.cz>
Date:   Fri, 17 Jan 2020 12:33:53 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Leonardo Bras <leonardo@...ux.ibm.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Allison Randal <allison@...utok.net>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Anshuman Khandual <anshuman.khandual@....com>,
        lantianyu1986@...il.com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

On Fri 17-01-20 11:57:59, David Hildenbrand wrote:
> Let's refactor that code. We want to check if we can offline memory
> blocks. Add a new function is_mem_section_offlineable() for that and
> make it call is_mem_section_offlineable() for each contained section.
> Within is_mem_section_offlineable(), add some more sanity checks and
> directly bail out if the section contains holes or if it spans multiple
> zones.

I didn't read the patch (yet) but I am wondering. If we want to touch
this code, can we simply always return true there? I mean whoever
depends on this check is racy and the failure can happen even after
the sysfs says good to go, right? The check is essentially as expensive
as calling the offlining code itself. So the only usecase I can think of
is a dumb driver to crawl over blocks and check which is removable and
try to hotremove it. But just trying to offline one block after another
is essentially going to achieve the same.

Or does anybody see any reasonable usecase that would break if we did
that unconditional behavior?
-- 
Michal Hocko
SUSE Labs