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@xxxxxxxxxx>
---
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@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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
