Re: [RFC PATCH 0/4] x86/mm/cpa: merge small mappings whenever possible

[Lu, Aaron]

On Thu, 2022-08-11 at 04:50 +0000, Hyeonggon Yoo wrote:
> 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!
> 

Hi Hyeonggon,

> +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/
> 

Thanks a lot for these info.

> > 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],

Cool, glad we have tried similar approach :-)

> and the biggest obstacle to this approach was: you need to be extremely sure
> that the page->nr_same_prot is ALWAYS correct.
> 

Yes indeed.

> 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
> 

For this specific case, it probably doesn't matter because kfence
intentionally uses set_memory_4k() for these pages and no merge shall
ever be done for them according to commit 1dc0da6e9ec0("x86, kfence:
enable KFENCE for x86").
(Kirill pointed out my current version has problem dealing with
set_memory_4k() but that is fixable).

> 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.

Thanks for the suggestion. I'll check how many callsites there are that
manipulate init_mm's page table outside of cpa() and then decides if it
is possible to do the hook and sync for set_pte/etc.

> 
> > 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
> > 
> >