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Date:   Tue, 17 Jan 2023 17:13:12 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Pasha Tatashin <pasha.tatashin@soleen.com>
Cc:     rppt@kernel.org, mhocko@suse.com, vbabka@suse.cz, david@redhat.com,
        quic_charante@quicinc.com, lizhe.67@bytedance.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm/page_ext: init page_ext early if there are no
 deferred struct pages
Message-Id: <20230117171312.13bdb3b2673bdac3c93c2ab7@linux-foundation.org>
In-Reply-To: <20230117204617.1553748-1-pasha.tatashin@soleen.com>
References: <20230117204617.1553748-1-pasha.tatashin@soleen.com>
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On Tue, 17 Jan 2023 20:46:17 +0000 Pasha Tatashin <pasha.tatashin@soleen.com> wrote:

> page_ext must be initialized after all struct pages are initialized.
> Therefore, page_ext is initialized after page_alloc_init_late(), and
> can optionally be initialized earlier via early_page_ext kernel
> parameter which as a side effect also disables deferred struct pages.
> 
> Allow to automatically init page_ext early when there are no deferred
> struct pages in order to be able to use page_ext during kernel boot
> and track for example page allocations early.

Compilation blows up with CONFIG_PAGE_EXTENSION=n.  I did

--- a/include/linux/page_ext.h~mm-page_ext-init-page_ext-early-if-there-are-no-deferred-struct-pages-fix
+++ a/include/linux/page_ext.h
@@ -92,6 +92,7 @@ static inline struct page_ext *page_ext_
 
 #else /* !CONFIG_PAGE_EXTENSION */
 struct page_ext;
+#define deferred_struct_pages (0)
 
 static inline bool early_page_ext_enabled(void)
 {

but it then blew up differently.