Re: [RFC v3 2/4] mm: move PG_slab flag to page_type

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 30, 2023 at 01:34:59PM +0900, Hyeonggon Yoo wrote:
> > Seems like quite some changes to page_type to accomodate SLAB, which is
> > hopefully going away soon(TM). Could we perhaps avoid that?
> 
> If it could be done with less changes, I'll try to avoid that.

Let me outline the idea I had for removing PG_slab:

Observe that PG_reserved and PG_slab are mutually exclusive.  Also,
if PG_reserved is set, no other flags are set.  If PG_slab is set, only
PG_locked is used.  Many of the flags are only for use by anon/page
cache pages (eg referenced, uptodate, dirty, lru, active, workingset,
waiters, error, owner_priv_1, writeback, mappedtodisk, reclaim,
swapbacked, unevictable, mlocked).

Redefine PG_reserved as PG_kernel.  Now we can use the other _15_
flags to indicate pagetype, as long as PG_kernel is set.  So, eg
PageSlab() can now be (page->flags & PG_type) == PG_slab where

#define PG_kernel	0x00001
#define PG_type		(PG_kernel | 0x7fff0)
#define PG_slab		(PG_kernel | 0x00010)
#define PG_reserved	(PG_kernel | 0x00020)
#define PG_buddy	(PG_kernel | 0x00030)
#define PG_offline	(PG_kernel | 0x00040)
#define PG_table	(PG_kernel | 0x00050)
#define PG_guard	(PG_kernel | 0x00060)

That frees up the existing PG_slab, lets us drop the page_type field
altogether and gives us space to define all the page types we might
want (eg PG_vmalloc)

We'll want to reorganise all the flags which are for anon/file pages
into a contiguous block.  And now that I think about it, vmalloc pages
can be mapped to userspace, so they can get marked dirty, so only
14 bits are available.  Maybe rearrange to ...

PG_locked	0x000001
PG_writeback	0x000002
PG_head		0x000004
PG_dirty	0x000008
PG_owner_priv_1	0x000010
PG_arch_1	0x000020
PG_private	0x000040
PG_waiters	0x000080
PG_kernel	0x000100
PG_referenced	0x000200
PG_uptodate	0x000400
PG_lru		0x000800
PG_active	0x001000
PG_workingset	0x002000
PG_error	0x004000
PG_private_2	0x008000
PG_mappedtodisk	0x010000
PG_reclaim	0x020000
PG_swapbacked	0x040000
PG_unevictable	0x080000
PG_mlocked	0x100000

... or something.  There are a number of constraints and it may take
a few iterations to get this right.  Oh, and if this is the layout
we use, then:

PG_type		0x1fff00
PG_reserved	(PG_kernel | 0x200)
PG_slab		(PG_kernel | 0x400)
PG_buddy	(PG_kernel | 0x600)
PG_offline	(PG_kernel | 0x800)
PG_table	(PG_kernel | 0xa00)
PG_guard	(PG_kernel | 0xc00)
PG_vmalloc	(PG_kernel | 0xe00)

This is going to make show_page_flags() more complex :-P

Oh, and while we're doing this, we should just make PG_mlocked
unconditional.  NOMMU doesn't need the extra space in page flags
(for what?  their large number of NUMA nodes?)