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Date: Fri, 24 May 2024 12:04:56 +0200
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Subject: Re: [PATCH RFC 2/6] mm: allow reuse of the lower 16bit of the page
 type with an actual type
To: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@infradead.org>,
 Mike Rapoport <rppt@kernel.org>, Minchan Kim <minchan@kernel.org>,
 Sergey Senozhatsky <senozhatsky@chromium.org>,
 Hyeonggon Yoo <42.hyeyoo@gmail.com>
References: <20240522210341.1030552-1-david@redhat.com>
 <20240522210341.1030552-3-david@redhat.com>
From: David Hildenbrand <david@redhat.com>
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On 22.05.24 23:03, David Hildenbrand wrote:
> As long as the owner sets a page type first, we can allow reuse of the
> lower 16bit! Restrict it to the head page.
> 
> We'll use that for zsmalloc next, to set a proper type while still
> reusing that field to store information that cannot go elsewhere for
> now.
> 
> Fear of running out of bits for storing the actual type? Actually, we
> don't need one bit per type, we could store a single value instead.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---

Likely the following might be better if we want to go down that path:


 From 39f198589039451d94166f3893dc939a919f74c7 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Wed, 22 May 2024 21:57:43 +0200
Subject: [PATCH] mm: allow reuse of the lower 18 bit of the page type with an
  actual type

As long as the owner sets a page type first, we can allow reuse of the
lower 18 bit: sufficient to store an offset into a 256 KiB page, which
is the maximum base page size we support. Restrict it to the head page.

We'll use that for zsmalloc next, to set a proper type while still
reusing that field to store information (offset into a base page) that
cannot go elsewhere for now.

Fear of running out of bits for storing the actual type? Actually, we
don't need one bit per type, we could store a single value instead.
Further, we could likely limit PAGE_TYPE_BASE to a single (highest) bit.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
  include/linux/mm_types.h   |  5 +++++
  include/linux/page-flags.h | 20 ++++++++++++--------
  2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 532a3030405d..437a62bed277 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -168,6 +168,11 @@ struct page {
  		 *
  		 * See page-flags.h for a list of page types which are currently
  		 * stored here.
+		 *
+		 * Owners of typed folios may reuse the lower 18 bit of the
+		 * head page page_type field after setting the page type,
+		 * but must reset these 18 bit to -1 before clearing the
+		 * page type.
  		 */
  		unsigned int page_type;
  	};
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 104078afe0b1..2f49c8b2f411 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -945,14 +945,18 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
   */
  
  #define PAGE_TYPE_BASE	0xf0000000
-/* Reserve		0x0000007f to catch underflows of _mapcount */
-#define PAGE_MAPCOUNT_RESERVE	-128
-#define PG_buddy	0x00000080
-#define PG_offline	0x00000100
-#define PG_table	0x00000200
-#define PG_guard	0x00000400
-#define PG_hugetlb	0x00000800
-#define PG_slab		0x00001000
+/*
+ * Reserve		0x0003ffff to catch underflows of _mapcount and
+ * allow owners that set a type to reuse the lower 18 bit for their own
+ * purposes.
+ */
+#define PAGE_MAPCOUNT_RESERVE	-262144
+#define PG_buddy	0x00040000
+#define PG_offline	0x00080000
+#define PG_table	0x00100000
+#define PG_guard	0x00200000
+#define PG_hugetlb	0x00400800
+#define PG_slab		0x00800000
  
  #define PageType(page, flag)						\
  	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
-- 
2.45.0


-- 
Cheers,

David / dhildenb