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From:   lizhe.67@bytedance.com
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Subject: Re: [PATCH v3] page_ext: introduce boot parameter early_page_ext
Date:   Thu, 25 Aug 2022 17:31:30 +0800
Message-Id: <20220825093130.98332-1-lizhe.67@bytedance.com>
In-Reply-To: <YwcgoZfw4RhZ1Bl6@dhcp22.suse.cz>
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On 25 Aug 2022 09:11:29 +0200, mhocko@suse.com wrote:
>On Thu 25-08-22 14:31:02, lizhe.67@bytedance.com wrote:
>> From: Li Zhe <lizhe.67@bytedance.com>
>> 
>> In 'commit 2f1ee0913ce5 ("Revert "mm: use early_pfn_to_nid in page_ext_init"")',
>> we call page_ext_init() after page_alloc_init_late() to avoid some panic
>> problem. It seems that we cannot track early page allocations in current
>> kernel even if page structure has been initialized early.
>> 
>> This patch introduce a new boot parameter 'early_page_ext' to resolve this
>> problem. If we pass it to kernel, function page_ext_init() will be moved
>> up and feature 'deferred initialization of struct pages' will be disabled.
>
>will be disabled to initialize the page allocator early and prevent from
>the OOM mentioned above.
>
>> It can help us to catch early page allocations. This is useful especially
>> when we find that the free memory value is not the same right after
>> different kernel booting.
>> 
>> Changelogs:
>> 
>> v1->v2:
>> - use a cmd line parameter to move up function page_ext_init() instead of
>>   using CONFIG_DEFERRED_STRUCT_PAGE_INIT
>> - fix oom problem[1]
>> 
>> v2->v3:
>> - make adjustments suggested by Michal Hocko
>> 
>> v1 patch: https://lore.kernel.org/lkml/Yv3r6Y1vh+6AbY4+@dhcp22.suse.cz/T/
>> v2 patch: https://lore.kernel.org/lkml/20220824065058.81051-1-lizhe.67@bytedance.com/T/
>> 
>> [1]: https://lore.kernel.org/linux-mm/YwHmXLu5txij+p35@xsang-OptiPlex-9020/
>
>the changelog is usually not part of the changelog and goes under ---

Thanks for correcting my mistake!

>> 
>> Suggested-by: Michal Hocko <mhocko@suse.com>
>> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
>
>I still have few comments below before I am going to ack. But this looks
>much better already.
>
>> ---
>>  Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
>>  include/linux/page_ext.h                        | 11 +++++++++++
>>  init/main.c                                     |  6 +++++-
>>  mm/page_alloc.c                                 |  2 ++
>>  mm/page_ext.c                                   | 12 ++++++++++++
>>  5 files changed, 36 insertions(+), 1 deletion(-)
>> 
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index d7f30902fda0..7b5726828ac0 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -1471,6 +1471,12 @@
>>  			Permit 'security.evm' to be updated regardless of
>>  			current integrity status.
>>  
>> +	early_page_ext [KNL] Boot-time early page_ext initializing option.
>> +			This boot parameter disables the deferred initialization
>> +			of struct page and move up function page_ext_init() in
>> +			order to catch early page allocations. Available with
>> +			CONFIG_PAGE_EXTENSION=y.
>
>For admins it would likely be more easier to understand something like
>following
>	early_page_ext [KNL] Enforces page_ext initialization to earlier
>			stages so cover more early boot allocations.
>			Please note that as side effect some 
>			optimizations might be disabled to achieve that
>			(e.g. parallelized memory initialization is
>			disabled) so the boot process might take longer,
>			especially on systems with a lot of memory.
>			Available with CONFIG_PAGE_EXTENSION=y

Great, I will use this description in my v4 patch. It is much more easier
for us to understand. Thanks!

>[...]
>> diff --git a/mm/page_ext.c b/mm/page_ext.c
>> index 3dc715d7ac29..bf4f2a12d7dc 100644
>> --- a/mm/page_ext.c
>> +++ b/mm/page_ext.c
>> @@ -85,6 +85,18 @@ unsigned long page_ext_size = sizeof(struct page_ext);
>>  
>>  static unsigned long total_usage;
>>  
>> +#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>> +bool early_page_ext __meminitdata;
>> +#else
>> +bool early_page_ext __meminitdata = true;
>> +#endif
>
>Why should default depend on DEFERRED_STRUCT_PAGE_INIT at all. This is
>just confusing and I do not see how it serves a purpose. We might grow
>more optimizations which would prefent early page_ext init.
>
>Let's just have default false and only enforce with the parameter. This
>is more predictable and easier to understand.

Yes, this is confusing. Without depending on DEFERRED_STRUCT_PAGE_INIT, the
logic here will be more clear. I will remove it from my v4 patch. Thanks!