On Mon, May 27, 2024 at 04:14:50PM +0200, David Hildenbrand 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 You say 18 here and 16 below. > 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. We could, but it's more instructions to check. > +++ 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 I think my original comment was misleading. This should be: * Reserve 0xffff0000 - 0xfffffffe to catch _mapcount underflow. How about we start at the top end and let people extend down? ie: #define PAGE_TYPE_BASE 0x80000000 #define PG_buddy 0x40000000 #define PG_offline 0x20000000 #define PG_table 0x10000000 #define PG_guard 0x08000000 #define PG_hugetlb 0x04000000 #define PG_slab 0x02000000 #define PAGE_MAPCOUNT_RESERVE (~0x0000ffff) Now we can see that we have 9 flags remaining, which should last until we can have proper memdesc typing.
