From: Catalin Marinas <catalin.marinas@arm.com>
To: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Baoquan He <bhe@redhat.com>,
Zhen Lei <thunder.leizhen@huawei.com>,
Ard Biesheuvel <ardb@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
Eric Biederman <ebiederm@xmission.com>,
Rob Herring <robh+dt@kernel.org>,
Frank Rowand <frowand.list@gmail.com>,
devicetree@vger.kernel.org, Dave Young <dyoung@redhat.com>,
Vivek Goyal <vgoyal@redhat.com>,
kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
Will Deacon <will@kernel.org>,
linux-arm-kernel@lists.infradead.org,
Jonathan Corbet <corbet@lwn.net>,
linux-doc@vger.kernel.org, Randy Dunlap <rdunlap@infradead.org>,
Feng Zhou <zhoufeng.zf@bytedance.com>,
Chen Zhou <dingguo.cz@antgroup.com>,
John Donnelly <John.p.donnelly@oracle.com>,
Dave Kleikamp <dave.kleikamp@oracle.com>,
liushixin <liushixin2@huawei.com>
Subject: Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory
Date: Tue, 21 Jun 2022 19:04:22 +0100 [thread overview]
Message-ID: <YrIIJkhKWSuAqkCx@arm.com> (raw)
In-Reply-To: <3f66323d-f371-b931-65fb-edfae0f01c88@huawei.com>
On Tue, Jun 21, 2022 at 02:24:01PM +0800, Kefeng Wang wrote:
> On 2022/6/21 13:33, Baoquan He wrote:
> > On 06/13/22 at 04:09pm, Zhen Lei wrote:
> > > If the crashkernel has both high memory above DMA zones and low memory
> > > in DMA zones, kexec always loads the content such as Image and dtb to the
> > > high memory instead of the low memory. This means that only high memory
> > > requires write protection based on page-level mapping. The allocation of
> > > high memory does not depend on the DMA boundary. So we can reserve the
> > > high memory first even if the crashkernel reservation is deferred.
> > >
> > > This means that the block mapping can still be performed on other kernel
> > > linear address spaces, the TLB miss rate can be reduced and the system
> > > performance will be improved.
> >
> > Ugh, this looks a little ugly, honestly.
> >
> > If that's for sure arm64 can't split large page mapping of linear
> > region, this patch is one way to optimize linear mapping. Given kdump
> > setting is necessary on arm64 server, the booting speed is truly
> > impacted heavily.
>
> Is there some conclusion or discussion that arm64 can't split large page
> mapping?
>
> Could the crashkernel reservation (and Kfence pool) be splited dynamically?
>
> I found Mark replay "arm64: remove page granularity limitation from
> KFENCE"[1],
>
> "We also avoid live changes from block<->table mappings, since the
> archtitecture gives us very weak guarantees there and generally requires
> a Break-Before-Make sequence (though IIRC this was tightened up
> somewhat, so maybe going one way is supposed to work). Unless it's
> really necessary, I'd rather not split these block mappings while
> they're live."
The problem with splitting is that you can end up with two entries in
the TLB for the same VA->PA mapping (e.g. one for a 4KB page and another
for a 2MB block). In the lucky case, the CPU will trigger a TLB conflict
abort (but can be worse like loss of coherency).
Prior to FEAT_BBM (added in ARMv8.4), such scenario was not allowed at
all, the software would have to unmap the range, TLBI, remap. With
FEAT_BBM (level 2), we can do this without tearing the mapping down but
we still need to handle the potential TLB conflict abort. The handler
only needs a TLBI but if it touches the memory range being changed it
risks faulting again. With vmap stacks and the kernel image mapped in
the vmalloc space, we have a small window where this could be handled
but we probably can't go into the C part of the exception handling
(tracing etc. may access a kmalloc'ed object for example).
Another option is to do a stop_machine() (if multi-processor at that
point), disable the MMUs, modify the page tables, re-enable the MMU but
it's also complicated.
--
Catalin
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next prev parent reply other threads:[~2022-06-21 18:04 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-13 8:09 [PATCH 0/5] arm64: kdump: Function supplement and performance optimization Zhen Lei
2022-06-13 8:09 ` [PATCH 1/5] arm64: kdump: Provide default size when crashkernel=Y,low is not specified Zhen Lei
2022-06-17 2:40 ` Baoquan He
2022-06-17 7:39 ` Leizhen (ThunderTown)
2022-06-17 8:26 ` Baoquan He
2022-06-13 8:09 ` [PATCH 2/5] arm64: kdump: Support crashkernel=X fall back to reserve region above DMA zones Zhen Lei
2022-06-17 4:16 ` Baoquan He
2022-06-13 8:09 ` [PATCH 3/5] arm64: kdump: Remove some redundant checks in map_mem() Zhen Lei
2022-06-20 7:42 ` Baoquan He
2022-06-13 8:09 ` [PATCH 4/5] arm64: kdump: Decide when to reserve crash memory in reserve_crashkernel() Zhen Lei
2022-06-13 8:09 ` [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory Zhen Lei
2022-06-21 5:33 ` Baoquan He
2022-06-21 6:24 ` Kefeng Wang
2022-06-21 9:27 ` Baoquan He
2022-06-21 18:04 ` Catalin Marinas [this message]
2022-06-22 8:35 ` Baoquan He
2022-06-23 14:07 ` Catalin Marinas
2022-06-27 2:52 ` Baoquan He
2022-06-27 9:17 ` Leizhen (ThunderTown)
2022-06-27 10:17 ` Baoquan He
2022-06-27 11:11 ` Leizhen (ThunderTown)
2022-06-22 12:03 ` Kefeng Wang
2022-06-23 10:27 ` Catalin Marinas
2022-06-23 14:23 ` Kefeng Wang
2022-06-21 7:56 ` Leizhen (ThunderTown)
2022-06-21 9:35 ` Baoquan He
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