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

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Date:   Thu, 2 Nov 2017 12:10:39 -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

>>
>> 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.
> 
> Can we make the init pattern to catch this?

Unfortunately, that is not easy: memset() gives us only one byte to play 
with, and if we use something else that will make CONFIG_VM_DEBUG 
unacceptably slow.

One byte is not enough to trigger the pattern that satisfy 
page_is_buddy() logic. I have tried it. With kexec, however it is more 
predictable: we use the same memory during boot to allocate vmemmap, and 
therefore the struct pages are more like "valid" struct pages from the 
previous boot.

> 
>>>>>> 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:
> 
> Could you post it please?
>   
>> 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
> 
> I think Andrew can drop it and replace by a different patch.
> 

The new patch is coming, I will test it on two machines where I observed 
the problem.

Thank you,
Pasha