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?)
