On Mon, Aug 08, 2022 at 10:56:45PM +0800, Aaron Lu wrote: > This is an early RFC. While all reviews are welcome, reviewing this code > now will be a waste of time for the x86 subsystem maintainers. I would, > however, appreciate a preliminary review from the folks on the to and cc > list. I'm posting it to the list in case anyone else is interested in > seeing this early version. > Hello Aaron! +Cc Mike Rapoport, who has been same problem. [1] There is also LPC discussion (with different approach on this problem) [2], [4] and performance measurement when all pages are 4K/2M. [3] [1] https://lore.kernel.org/linux-mm/20220127085608.306306-1-rppt@xxxxxxxxxx/ [2] https://www.youtube.com/watch?v=egC7ZK4pcnQ [3] https://lpc.events/event/11/contributions/1127/attachments/922/1792/LPC21%20Direct%20map%20management%20.pdf [4] https://lwn.net/Articles/894557/ > Dave Hansen: I need your ack before this goes to the maintainers. > > Here it goes: > > On x86_64, Linux has direct mapping of almost all physical memory. For > performance reasons, this mapping is usually set as large page like 2M > or 1G per hardware's capability with read, write and non-execute > protection. > > There are cases where some pages have to change their protection to RO > and eXecutable, like pages that host module code or bpf prog. When these > pages' protection are changed, the corresponding large mapping that > cover these pages will have to be splitted into 4K first and then > individual 4k page's protection changed accordingly, i.e. unaffected > pages keep their original protection as RW and NX while affected pages' > protection changed to RO and X. > > There is a problem due to this split: the large mapping will remain > splitted even after the affected pages' protection are changed back to > RW and NX, like when the module is unloaded or bpf progs are freed. > After system runs a long time, there can be more and more large mapping > being splitted, causing more and more dTLB misses and overall system > performance getting hurt[1]. > > For this reason, people tried some techniques to reduce the harm of > large mapping beling splitted, like bpf_prog_pack[2] which packs > multiple bpf progs into a single page instead of allocating and changing > one page's protection for each bpf prog. This approach made large > mapping split happen much fewer. > > This patchset addresses this problem in another way: it merges > splitted mappings back to a large mapping when protections of all entries > of the splitted small mapping page table become same again, e.g. when the > page whose protection was changed to RO+X now has its protection changed > back to RW+NX due to reasons like module unload, bpf prog free, etc. and > all other entries' protection are also RW+NX. > I tried very similar approach few months ago (for toy implementation) [5], and the biggest obstacle to this approach was: you need to be extremely sure that the page->nr_same_prot is ALWAYS correct. For example, in arch/x86/include/asm/kfence.h [6], it clears and set _PAGE_PRESENT without going through CPA, which can simply break the count. [5] https://github.com/hygoni/linux/tree/merge-mapping-v1r3 [6] https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/kfence.h#L56 I think we may need to hook set_pte/set_pmd/etc and use proper synchronization primitives when changing init_mm's page table to go further on this approach. > One final note is, with features like bpf_prog_pack etc., there can be > much fewer large mapping split IIUC; also, this patchset can not help > when the page which has its protection changed keeps in use. So my take > on this large mapping split problem is: to get the most value of keeping > large mapping intact, features like bpf_prog_pack is important. This > patchset can help to further reduce large mapping split when in use page > that has special protection set finally gets released. > > [1]: http://lkml.kernel.org/r/CAPhsuW4eAm9QrAxhZMJu-bmvHnjWjuw86gFZzTHRaMEaeFhAxw@xxxxxxxxxxxxxx > [2]: https://lore.kernel.org/lkml/20220204185742.271030-1-song@xxxxxxxxxx/ > > Aaron Lu (4): > x86/mm/cpa: restore global bit when page is present > x86/mm/cpa: merge splitted direct mapping when possible > x86/mm/cpa: add merge event counter > x86/mm/cpa: add a test interface to split direct map > > arch/x86/mm/pat/set_memory.c | 411 +++++++++++++++++++++++++++++++++- > include/linux/mm_types.h | 6 + > include/linux/page-flags.h | 6 + > include/linux/vm_event_item.h | 2 + > mm/vmstat.c | 2 + > 5 files changed, 420 insertions(+), 7 deletions(-) > > -- > 2.37.1 > >
