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Date:   Wed, 12 May 2021 16:57:25 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Ilias Apalodimas <ilias.apalodimas@linaro.org>
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Subject: Re: [PATCH net-next v4 1/4] mm: add a signature in struct page
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On Tue, May 11, 2021 at 05:25:36PM +0300, Ilias Apalodimas wrote:
> Nope not at all, either would work. we'll switch to that

You'll need something like this because of the current use of
page->index to mean "pfmemalloc".

From ecd6d912056a21bbe55d997c01f96b0b8b9fbc31 Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Fri, 16 Apr 2021 18:12:33 -0400
Subject: [PATCH] mm: Indicate pfmemalloc pages in compound_head

The net page_pool wants to use a magic value to identify page pool pages.
The best place to put it is in the first word where it can be clearly a
non-pointer value.  That means shifting dma_addr up to alias with ->index,
which means we need to find another way to indicate page_is_pfmemalloc().
Since page_pool doesn't want to set its magic value on pages which are
pfmemalloc, we can use bit 1 of compound_head to indicate that the page
came from the memory reserves.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/mm.h       | 12 +++++++-----
 include/linux/mm_types.h |  7 +++----
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index bd21864449bf..4f9b2007efad 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1670,10 +1670,12 @@ struct address_space *page_mapping(struct page *page);
 static inline bool page_is_pfmemalloc(const struct page *page)
 {
 	/*
-	 * Page index cannot be this large so this must be
-	 * a pfmemalloc page.
+	 * This is not a tail page; compound_head of a head page is unused
+	 * at return from the page allocator, and will be overwritten
+	 * by callers who do not care whether the page came from the
+	 * reserves.
 	 */
-	return page->index == -1UL;
+	return page->compound_head & 2;
 }
 
 /*
@@ -1682,12 +1684,12 @@ static inline bool page_is_pfmemalloc(const struct page *page)
  */
 static inline void set_page_pfmemalloc(struct page *page)
 {
-	page->index = -1UL;
+	page->compound_head = 2;
 }
 
 static inline void clear_page_pfmemalloc(struct page *page)
 {
-	page->index = 0;
+	page->compound_head = 0;
 }
 
 /*
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5aacc1c10a45..1352e278939b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -96,10 +96,9 @@ struct page {
 			unsigned long private;
 		};
 		struct {	/* page_pool used by netstack */
-			/**
-			 * @dma_addr: might require a 64-bit value on
-			 * 32-bit architectures.
-			 */
+			unsigned long pp_magic;
+			struct page_pool *pp;
+			unsigned long _pp_mapping_pad;
 			unsigned long dma_addr[2];
 		};
 		struct {	/* slab, slob and slub */
-- 
2.30.2