On Mon, 17 Jun 2024 15:14:58 -0700 Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> wrote:
> > -#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
> > +enum pagetype {
> > + /*
> > + * Reserve 0xffff0000 - 0xfffffffe to catch _mapcount underflows and
> > + * allow owners that set a type to reuse the lower 16 bit for their own
> > + * purposes.
> > + */
>
> This comment is a bit out of place now, because it refers to David's
> change which has not yet been applied. Maybe it should continue to read
>
> "Reserve 0x0000007f to catch underflows of _mapcount"
>
> until "mm: allow reuse of the lower 16 bit of the page type with an
> actual type" changes it?
Updated, thanks.
The post-Stephen code is now
/*
* For pages that are never mapped to userspace,
* page_type may be used. Because it is initialised to -1, we invert the
* sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
* __ClearPageFoo *sets* the bit used for PageFoo. We reserve a few high and
* low bits so that an underflow or overflow of _mapcount won't be
* mistaken for a page type value.
*/
enum pagetype {
PG_buddy = 0x00000080,
PG_offline = 0x00000100,
PG_table = 0x00000200,
PG_guard = 0x00000400,
PG_hugetlb = 0x00000800,
PG_slab = 0x00001000,
PAGE_TYPE_BASE = 0xf0000000,
/* Reserve 0x0000007f to catch underflows of _mapcount */
PAGE_MAPCOUNT_RESERVE = -128,
};
And the post-David code is now:
/*
* For pages that are never mapped to userspace,
* page_type may be used. Because it is initialised to -1, we invert the
* sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
* __ClearPageFoo *sets* the bit used for PageFoo. We reserve a few high and
* low bits so that an underflow or overflow of _mapcount won't be
* mistaken for a page type value.
*/
enum pagetype {
PG_buddy = 0x40000000,
PG_offline = 0x20000000,
PG_table = 0x10000000,
PG_guard = 0x08000000,
PG_hugetlb = 0x04008000,
PG_slab = 0x02000000,
PAGE_TYPE_BASE = 0x80000000,
/*
* Reserve 0xffff0000 - 0xfffffffe to catch _mapcount underflows and
* allow owners that set a type to reuse the lower 16 bit for their own
* purposes.
*/
PAGE_MAPCOUNT_RESERVE = ~0x0000ffff,
};
