[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ZlSmNzgkeYaCrPWc@casper.infradead.org>
Date: Mon, 27 May 2024 16:26:47 +0100
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>,
Mike Rapoport <rppt@...nel.org>, Minchan Kim <minchan@...nel.org>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH v1 2/6] mm: allow reuse of the lower 16 bit of the page
type with an actual type
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.
Powered by blists - more mailing lists