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

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Date:   Thu, 2 Nov 2017 10:16:49 -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

>>>> 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...

Yes, but as I said, unfortunately memset(1) with CONFIG_VM_DEBUG does 
not catch this case. So, when CONFIG_VM_DEBUG is enabled kexec reboots 
without issues.

>   
>>>> 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.
> 

Ok, so should I submit a new patch with two loops? (The logic within 
loops is going to be the same:

if (!pfn_valid_within(pfn)) {
} else if (!(pfn & nr_pgmask) && !pfn_valid(pfn)) {
} else if (!meminit_pfn_in_nid(pfn, nid, &nid_init_state)) {
} else if (page && (pfn & nr_pgmask)) {

This fix was already added into mm-tree as
mm-deferred_init_memmap-improvements-fix-2.patch

Thank you,
Pasha