linux-kernel - Re: [PATCH v1 1/1] mm: buddy page accessed before initialized

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Message-ID: <20171102140830.z5uqmrurb6ohfvlj@dhcp22.suse.cz>
Date:   Thu, 2 Nov 2017 15:08:30 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/1] mm: buddy page accessed before initialized

On Thu 02-11-17 10:00:59, Pavel Tatashin wrote:
> 
> 
> On 11/02/2017 09:54 AM, Michal Hocko wrote:
> > On Thu 02-11-17 09:39:58, Pavel Tatashin wrote:
> > [...]
> > > Hi Michal,
> > > 
> > > Previously as before my project? That is because memory for all struct pages
> > > was always zeroed in memblock, and in __free_one_page() page_is_buddy() was
> > > always returning false, thus we never tried to incorrectly remove it from
> > > the list:
> > > 
> > > 837			list_del(&buddy->lru);
> > > 
> > > Now, that memory is not zeroed, page_is_buddy() can return true after kexec
> > > when memory is dirty (unfortunately memset(1) with CONFIG_VM_DEBUG does not
> > > catch this case). And proceed further to incorrectly remove buddy from the
> > > list.
> > 
> > OK, I thought this was a regression from one of the recent patches. So
> > the problem is not new. Why don't we see the same problem during the
> > standard boot?
> 
> Because, I believe, BIOS is zeroing all the memory for us.

I thought you were runnning with the debugging which poisons all the
allocated memory...
 
> > > This is why we must initialize the computed buddy page beforehand.
> > 
> > Ble, this is really ugly. I will think about it more.
> > 
> 
> Another approach that I considered is to split loop inside
> deferred_init_range() into two loops: one where we initialize pages by
> calling __init_single_page(), another where we free them to buddy allocator
> by calling deferred_free_range().

Yes, that would make much more sense to me.

-- 
Michal Hocko
SUSE Labs