Re: [PATCH v1 26] mm allow reuse of the lower 16 bit of the page type with an actual type

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 29.05.24 17:55, wang wei wrote:
---
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@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 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

Every PG_XX occupies one bit in my understanding.   But why PG_hugetlb occupies two bits?

Because it's wrong (although not harmful). Same issue in v2, fat fingers.

Thanks for pointing that out!

--
Cheers,

David / dhildenb





[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Bugtraq]     [Linux OMAP]     [Linux MIPS]     [eCos]     [Asterisk Internet PBX]     [Linux API]

  Powered by Linux