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. > 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. Not sure that doing the scan in each set_memory call won't cause an overall slowdown. > 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 > > > > arch/Kconfig | 7 ++ > > arch/x86/Kconfig | 1 + > > arch/x86/kernel/module.c | 2 +- > > include/linux/gfp.h | 13 +++- > > include/linux/mmzone.h | 11 +++ > > include/trace/events/mmflags.h | 3 +- > > mm/internal.h | 2 +- > > mm/page_alloc.c | 129 ++++++++++++++++++++++++++++++++- > > mm/secretmem.c | 8 +- > > 9 files changed, 162 insertions(+), 14 deletions(-) > > > > > > base-commit: e783362eb54cd99b2cac8b3a9aeac942e6f6ac07 > > -- > > 2.34.1 > > > > -- > Thanks, > Hyeonggon -- Sincerely yours, Mike.
