From: Hyeonggon Yoo <42.hyeyoo@gmail.com>
To: Aaron Lu <aaron.lu@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>,
Rick Edgecombe <rick.p.edgecombe@intel.com>,
Song Liu <song@kernel.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Mike Rapoport <rppt@kernel.org>,
Hyeonggon Yoo <42.hyeyoo@gmail.com>
Subject: Re: [RFC PATCH 0/4] x86/mm/cpa: merge small mappings whenever possible
Date: Thu, 11 Aug 2022 04:50:44 +0000 [thread overview]
Message-ID: <YvSKpCWhAz+2Y220@ip-172-31-24-42.ap-northeast-1.compute.internal> (raw)
In-Reply-To: <20220808145649.2261258-1-aaron.lu@intel.com>
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@kernel.org/
[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@mail.gmail.com
> [2]: https://lore.kernel.org/lkml/20220204185742.271030-1-song@kernel.org/
>
> 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
>
>
next prev parent reply other threads:[~2022-08-11 4:50 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-08 14:56 [RFC PATCH 0/4] x86/mm/cpa: merge small mappings whenever possible Aaron Lu
2022-08-08 14:56 ` [RFC PATCH 1/4] x86/mm/cpa: restore global bit when page is present Aaron Lu
2022-08-11 5:21 ` Hyeonggon Yoo
2022-08-11 8:16 ` Lu, Aaron
2022-08-11 11:30 ` Hyeonggon Yoo
2022-08-11 12:28 ` Aaron Lu
2022-08-08 14:56 ` [RFC PATCH 2/4] x86/mm/cpa: merge splitted direct mapping when possible Aaron Lu
2022-08-08 14:56 ` [RFC PATCH 3/4] x86/mm/cpa: add merge event counter Aaron Lu
2022-08-08 14:56 ` [TEST NOT_FOR_MERGE 4/4] x86/mm/cpa: add a test interface to split direct map Aaron Lu
2022-08-09 10:04 ` [RFC PATCH 0/4] x86/mm/cpa: merge small mappings whenever possible Kirill A. Shutemov
2022-08-09 14:58 ` Aaron Lu
2022-08-09 17:56 ` Kirill A. Shutemov
2022-08-11 4:50 ` Hyeonggon Yoo [this message]
2022-08-11 7:50 ` Lu, Aaron
2022-08-13 16:05 ` Mike Rapoport
2022-08-16 6:33 ` Aaron Lu
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