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

Hyeonggon Yoo
2023-02-03T16:00:28Z

On Mon, Jan 30, 2023 at 05:11:48AM +0000, Matthew Wilcox wrote:
> 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 PG_kernel is a new special flag, I thought it indicates
"not usermappable pages", but considering PG_vmalloc it's not.

> So, eg
> PageSlab() can now be (page->flags & PG_type) == PG_slab where

But if PG_xxx and PG_slab shares same bit, PG_xxx would be confused?

> #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

I think slab still needs PG_head,
but it seems to be okay with this layout.
(but these assumpstions are better documented, I think)

> 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)

what is PG_vmalloc for, is it just an example for
explaining possible layout?

> 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?)