On Sat, Apr 30, 2022 at 01:44:16PM +0000, Hyeonggon Yoo wrote: > On Tue, Apr 26, 2022 at 06:21:57PM +0300, Mike Rapoport wrote: > > Hello Hyeonggon, > > > > On Tue, Apr 26, 2022 at 05:54:49PM +0900, Hyeonggon Yoo wrote: > > > On Thu, Jan 27, 2022 at 10:56:05AM +0200, Mike Rapoport wrote: > > > > From: Mike Rapoport <rppt@xxxxxxxxxxxxx> > > > > > > > > Hi, > > > > > > > > This is a second attempt to make page allocator aware of the direct map > > > > layout and allow grouping of the pages that must be mapped at PTE level in > > > > the direct map. > > > > > > > > > > Hello mike, It may be a silly question... > > > > > > Looking at implementation of set_memory*(), they only split > > > PMD/PUD-sized entries. But why not _merge_ them when all entries > > > have same permissions after changing permission of an entry? > > > > > > I think grouping __GFP_UNMAPPED allocations would help reducing > > > direct map fragmentation, but IMHO merging split entries seems better > > > to be done in those helpers than in page allocator. > > > > Maybe, I didn't got as far as to try merging split entries in the direct > > map. IIRC, Kirill sent a patch for collapsing huge pages in the direct map > > some time ago, but there still was something that had to initiate the > > collapse. > > But in this case buddy allocator's view of direct map is quite limited. > It cannot merge 2M entries to 1G entry as it does not support > big allocations. Also it cannot merge entries of pages freed in boot process > as they weren't allocated from page allocator. > > And it will become harder when pages in MIGRATE_UNMAPPED is borrowed > from another migrate type.... > > So it would be nice if we can efficiently merge mappings in > change_page_attr_set(). this approach can handle cases above. > > I think in this case grouping allocations and merging mappings > should be done separately. I've added the provision to merge the mappings in __free_one_page() because at that spot we know for sure we can replace multiple PTEs with a single PMD. I'm not saying there should be no additional mechanism for collapsing direct map pages, but I don't know when and how it should be invoked. > > > For example: > > > 1) set_memory_ro() splits 1 RW PMD entry into 511 RW PTE > > > entries and 1 RO PTE entry. > > > > > > 2) before freeing the pages, we call set_memory_rw() and we have > > > 512 RW PTE entries. Then we can merge it to 1 RW PMD entry. > > > > For this we need to check permissions of all 512 pages to make sure we can > > use a PMD entry to map them. > > Of course that may be slow. Maybe one way to optimize this is using some bits > in struct page, something like: each bit of page->direct_map_split (unsigned long) > is set when at least one entry in (PTRS_PER_PTE = 512)/(BITS_PER_LONG = 64) = 8 entries > has special permissions. > > Then we just need to set the corresponding bit when splitting mappings and > iterate 8 entries when changing permission back again. (and then unset the bit when 8 entries has > usual permissions). we can decide to merge by checking if page->direct_map_split is zero. > > When scanning, 8 entries would fit into one cacheline. > > Any other ideas? > > > Not sure that doing the scan in each set_memory call won't cause an overall > > slowdown. > > I think we can evaluate it by measuring boot time and bpf/module > load/unload time. > > Is there any other workload that is directly affected > by performance of set_memory*()? > > > > 3) after 2) we can do same thing about PMD-sized entries > > > and merge them into 1 PUD entry if 512 PMD entries have > > > same permissions. > > > [...] > > > > Mike Rapoport (3): > > > > mm/page_alloc: introduce __GFP_UNMAPPED and MIGRATE_UNMAPPED > > > > mm/secretmem: use __GFP_UNMAPPED to allocate pages > > > > EXPERIMENTAL: x86/module: use __GFP_UNMAPPED in module_alloc > > > -- > > > Thanks, > > > Hyeonggon > > > > -- > > Sincerely yours, > > Mike. -- Sincerely yours, Mike.
