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Message-ID: <20240527141454.113132-3-david@redhat.com>
Date: Mon, 27 May 2024 16:14:50 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
Mike Rapoport <rppt@...nel.org>,
Minchan Kim <minchan@...nel.org>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: [PATCH v1 2/6] mm: allow reuse of the lower 16 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 64 KiB page, which
is the maximum base page size in *common* configurations (ignoring the
256 KiB variant). 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@...hat.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 6b2aeba792c4..598cfedbbfa0 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -157,6 +157,11 @@ 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 16 bit of the
+ * head page page_type field after setting the page type,
+ * but must reset these 16 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..b43e380ffa0b 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 0x0000ffff to catch underflows of _mapcount and
+ * allow owners that set a type to reuse the lower 16 bit for their own
+ * purposes.
+ */
+#define PAGE_MAPCOUNT_RESERVE -65536
+#define PG_buddy 0x00010000
+#define PG_offline 0x00020000
+#define PG_table 0x00040000
+#define PG_guard 0x00080000
+#define PG_hugetlb 0x00100800
+#define PG_slab 0x00200000
#define PageType(page, flag) \
((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
--
2.45.1
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