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Date:   Thu, 18 Aug 2022 09:36:09 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     lizhe.67@bytedance.com
Cc:     akpm@linux-foundation.org, mhiramat@kernel.org, vbabka@suse.cz,
        keescook@chromium.org, Jason@zx2c4.com, mark-pk.tsai@mediatek.com,
        rostedt@goodmis.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, lizefan.x@bytedance.com, yuanzhu@bytedance.com
Subject: Re: [PATCH] page_ext: move up page_ext_init() to catch early page
 allocation if DEFERRED_STRUCT_PAGE_INIT is n
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On Mon 15-08-22 20:09:54, 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 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?

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).
-- 
Michal Hocko
SUSE Labs