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From:   lizhe.67@bytedance.com
To:     mhocko@suse.com
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Subject: Re: [PATCH] page_ext: move up page_ext_init() to catch early page allocation if DEFERRED_STRUCT_PAGE_INIT is n
Date:   Sat, 20 Aug 2022 09:02:57 +0800
Message-Id: <20220820010257.11488-1-lizhe.67@bytedance.com>
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On 2022-08-18 7:36 UTC, mhocko@suse.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 move up page_ext_init() to catch early page allocations when
>> DEFERRED_STRUCT_PAGE_INIT is n. After this patch, we only need to turn
>> DEFERRED_STRUCT_PAGE_INIT to n then we are able to analyze the early page
>> allocations. This is useful especially when we find that the free memory
>> value is not the same right after different kernel booting.
>
>is this actually useful in practice? I mean who is going to disable
>DEFERRED_STRUCT_PAGE_INIT and recompile the kernel for debugging early
>allocations?

Yes it is useful. We use this method to catch the difference of early
page allocations between two kernel.

> I do see how debugging those early allocations might be useful but that
> would require a boot time option to be practical IMHO. Would it make
> sense to add a early_page_ext parameter which would essentially disable
> the deferred ipage initialization. That should be quite trivial to
> achieve (just hook into defer_init AFAICS).

It is a good idea. A cmdline parameter is a flexible and dynamic method for
us to decide whether to defer page's and page_ext's initilization. For
comparison, this patch provides a static method to decide whether to defer
page's and page_ext's initilization. They are not conflicting. My next
work is trying to achieve your idea.
--
Li Zhe