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?)
