linux-kernel - Re: [PATCH AUTOSEL 4.14 62/95] mm, memory_hotplug: initialize struct pages for the full memory section

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Message-ID: <20190507173121.GR31017@dhcp22.suse.cz>
Date:   Tue, 7 May 2019 19:31:21 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Sasha Levin <sashal@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Mikhail Zaslonko <zaslonko@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Pasha Tatashin <Pavel.Tatashin@...rosoft.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sasha Levin <alexander.levin@...rosoft.com>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH AUTOSEL 4.14 62/95] mm, memory_hotplug: initialize struct
 pages for the full memory section

On Tue 07-05-19 10:15:19, Linus Torvalds wrote:
> On Tue, May 7, 2019 at 10:02 AM Sasha Levin <sashal@...nel.org> wrote:
> >
> > I got it wrong then. I'll fix it up and get efad4e475c31 in instead.

This patch is not marked for stable backports for good reasons.

> 
> Careful. That one had a bug too, and we have 891cb2a72d82 ("mm,
> memory_hotplug: fix off-by-one in is_pageblock_removable").
> 
> All of these were *horribly* and subtly buggy, and might be
> intertwined with other issues. And only trigger on a few specific
> machines where the memory map layout is just right to trigger some
> special case or other, and you have just the right config.

Yes, the code turned out to be much more tricky than we thought. There
were several assumptions about alignment etc. Something that is really
hard to test for because HW breaking those assumptions is rare. So I
would discourage picking up some random patches in the memory hotplug
for stable. Each patch needs a very careful consideration. In any case
we really try hard to keep Fixes: tag accurate so at least those should
be scanned.

-- 
Michal Hocko
SUSE Labs