linux-kernel - Re: [PATCH] page_ext: move up page_ext_init() to catch early page allocation if DEFERRED_STRUCT_PAGE

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Date:   Mon, 22 Aug 2022 09:00:33 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     lizhe.67@...edance.com, mhocko@...e.com
Cc:     Jason@...c4.com, akpm@...ux-foundation.org, keescook@...omium.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        lizefan.x@...edance.com, mark-pk.tsai@...iatek.com,
        mhiramat@...nel.org, rostedt@...dmis.org, yuanzhu@...edance.com
Subject: Re: [PATCH] page_ext: move up page_ext_init() to catch early page
 allocation if DEFERRED_STRUCT_PAGE_INIT is n

On 8/20/22 03:02, lizhe.67@...edance.com wrote:
> On 2022-08-18 7:36 UTC, mhocko@...e.com wrote:
>>> From: Li Zhe <lizhe.67@...edance.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.

As we already have to pass page_owner=on parameter to enable the page
allocation tracking in the first place, maybe that alone could also disable
deffered init, and no need for another parameter?

> --
> Li Zhe