Re: [PATCH] page_ext: move up page_ext_init() to catch early page allocation if DEFERRED_STRUCT_PAGE_INIT is n

[lizhe.67@bytedance.com]

On 2022-08-22  7:08 UTC, mhocko@suse.com wrote:
>On Sat 20-08-22 09:02:57, lizhe.67@bytedance.com wrote:
>> 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 was not questioning the functionality itself but the way how it is
>achieved. Recompiling the kernel to achieve debuggability has proven to
>be really a bad approach historically. Most people are using
>pre-compiled kernels these days.
>
>> > 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.
>
>They are not conflicting but this patch adds ifdefs and additional code
>that needs compile time testing with different options set. I.e. it adds
>maintenance burden for something that can be achieved by better means.
>So if you are ok to work on the runtime knob then I would propose to
>drop this patch from the mm tree and replace it by a trivial patch to
>allow early boot debugging by a cmd line parameter.

Yes you are right. Recompiling the kernel is not a clever method. I will send
another patch with a cmd line parameter 'early_page_ext' to achieve this idea.
Thanks for your advice.