On Fri, Feb 03, 2023 at 04:00:08PM +0000, Hyeonggon Yoo wrote: > 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. Right, it means "The kernel allocated this page for its own purposes; what that purpose is might be available by looking at PG_type". ie it's not-anon, not-page-cache. > > 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? Correct. Ideally those tests wouldn't be used on arbitrary pages, only pages which are already confirmed to be anon or file. I suspect we haven't been super-careful about that in the past, and so there would be some degree of "Oh, we need to fix this up". But flags like PG_mappedtodisk, PG_mlocked, PG_unevictable, PG_workingset should be all gated behind "We know this is anon/file". > > #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) Yes, slab need PG_head so it knows whether this is a multi-page slab or not. I forgot to mention it above as a bit that slab needs, but I put it in the low bits here. > > 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? I really want to mark pages as being allocated from vmalloc. It's one of the things we could do to make debugging better.
