From 675b73aaa7718e93e9f2492a3b9cc417f9e820b4 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Mon, 7 Nov 2022 15:48:27 -0800 Subject: [PATCH 1/4] mm: introduce 'encoded' page pointers with embedded extra bits We already have this notion in parts of the MM code (see the mlock code with the LRU_PAGE and NEW_PAGE) bits, but I'm going to introduce a new case, and I refuse to do the same thing we've done before where we just put bits in the raw pointer and say it's still a normal pointer. So this introduces a 'struct encoded_page' pointer that cannot be used for anything else than to encode a real page pointer and a couple of extra bits in the low bits. That way the compiler can trivially track the state of the pointer and you just explicitly encode and decode the extra bits. Signed-off-by: Linus Torvalds --- include/linux/mm_types.h | 33 ++++++++++++++++++++++++++++++++- 1 file changed, 32 insertions(+), 1 deletion(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 500e536796ca..b5cffd250784 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -67,7 +67,7 @@ struct mem_cgroup; #ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE #define _struct_page_alignment __aligned(2 * sizeof(unsigned long)) #else -#define _struct_page_alignment +#define _struct_page_alignment __aligned(sizeof(unsigned long)) #endif struct page { @@ -241,6 +241,37 @@ struct page { #endif } _struct_page_alignment; +/** + * struct encoded_page - a nonexistent type marking this pointer + * + * An 'encoded_page' pointer is a pointer to a regular 'struct page', but + * with the low bits of the pointer indicating extra context-dependent + * information. Not super-common, but happens in mmu_gather and mlock + * handling, and this acts as a type system check on that use. + * + * We only really have two guaranteed bits in general, although you could + * play with 'struct page' alignment (see CONFIG_HAVE_ALIGNED_STRUCT_PAGE) + * for more. + * + * Use the supplied helper functions to endcode/decode the pointer and bits. + */ +struct encoded_page; +#define ENCODE_PAGE_BITS 3ul +static inline struct encoded_page *encode_page(struct page *page, unsigned long flags) +{ + return (struct encoded_page *)(flags | (unsigned long)page); +} + +static inline bool encoded_page_flags(struct encoded_page *page) +{ + return ENCODE_PAGE_BITS & (unsigned long)page; +} + +static inline struct page *encoded_page_ptr(struct encoded_page *page) +{ + return (struct page *)(~ENCODE_PAGE_BITS & (unsigned long)page); +} + /** * struct folio - Represents a contiguous set of bytes. * @flags: Identical to the page flags. -- 2.37.1.289.g45aa1e5c72.dirty