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

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Date:   Thu, 2 Nov 2017 10:00:59 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
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 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.

> 
>> 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().

Pasha