Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

From: Catalin Marinas <catalin.marinas@arm.com>
To: "guanghui.fgh" <guanghuifeng@linux.alibaba.com>
Cc: Mike Rapoport <rppt@kernel.org>, Will Deacon <will@kernel.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	baolin.wang@linux.alibaba.com, akpm@linux-foundation.org,
	david@redhat.com, jianyong.wu@arm.com, james.morse@arm.com,
	quic_qiancai@quicinc.com, christophe.leroy@csgroup.eu,
	jonathan@marek.ca, mark.rutland@arm.com,
	thunder.leizhen@huawei.com, anshuman.khandual@arm.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, geert+renesas@glider.be,
	linux-mm@kvack.org, yaohongbo@linux.alibaba.com,
	alikernel-developer@linux.alibaba.com
Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation
Date: Wed, 6 Jul 2022 16:40:56 +0100	[thread overview]
Message-ID: <YsWtCLIG2qKETqmq@arm.com> (raw)
In-Reply-To: <9974bea5-4db9-0104-c9c9-d9b49c390f1b@linux.alibaba.com>

On Wed, Jul 06, 2022 at 11:18:22PM +0800, guanghui.fgh wrote:
> 在 2022/7/6 21:54, Mike Rapoport 写道:
> > One thing I can think of is to only remap the crash kernel memory if it is
> > a part of an allocation that exactly fits into one ore more PUDs.
> > 
> > Say, in reserve_crashkernel() we try the memblock_phys_alloc() with
> > PUD_SIZE as alignment and size rounded up to PUD_SIZE. If this allocation
> > succeeds, we remap the entire area that now contains only memory allocated
> > in reserve_crashkernel() and free the extra memory after remapping is done.
> > If the large allocation fails, we fall back to the original size and
> > alignment and don't allow unmapping crash kernel memory in
> > arch_kexec_protect_crashkres().
> 
> There is a new method.
> I think we should use the patch v3(similar but need add some changes)
> 
> 1.We can walk crashkernle block/section pagetable,
> [[[(keep the origin block/section mapping valid]]]
> rebuild the pte level page mapping for the crashkernel mem
> rebuild left & right margin mem(which is in same block/section mapping but
> out of crashkernel mem) with block/section mapping
> 
> 2.'replace' the origin block/section mapping by new builded mapping
> iterately
> 
> With this method, all the mem mapping keep valid all the time.

As I already commented on one of your previous patches, this is not
allowed by the architecture. If FEAT_BBM is implemented (ARMv8.4 I
think), the worst that can happen is a TLB conflict abort and the
handler should invalidate the TLBs and restart the faulting instruction,
assuming the handler won't try to access the same conflicting virtual
address. Prior to FEAT_BBM, that's not possible as the architecture does
not describe a precise behaviour of conflicting TLB entries (you might
as well get the TLB output of multiple entries being or'ed together).

-- 
Catalin