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

From: Mike Rapoport <rppt@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>,
	"guanghui.fgh" <guanghuifeng@linux.alibaba.com>,
	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: Tue, 5 Jul 2022 23:45:40 +0300	[thread overview]
Message-ID: <YsSi9HAOOzbPYN+w@kernel.org> (raw)
In-Reply-To: <YsRvPTORdvIwzShL@arm.com>

On Tue, Jul 05, 2022 at 06:05:01PM +0100, Catalin Marinas wrote:
> On Tue, Jul 05, 2022 at 06:57:53PM +0300, Mike Rapoport wrote:
> > On Tue, Jul 05, 2022 at 04:34:09PM +0100, Catalin Marinas wrote:
> > > On Tue, Jul 05, 2022 at 06:02:02PM +0300, Mike Rapoport wrote:
> > > > +void __init remap_crashkernel(void)
> > > > +{
> > > > +#ifdef CONFIG_KEXEC_CORE
> > > > +	phys_addr_t start, end, size;
> > > > +	phys_addr_t aligned_start, aligned_end;
> > > > +
> > > > +	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> > > > +	    return;
> > > > +
> > > > +	if (!crashk_res.end)
> > > > +	    return;
> > > > +
> > > > +	start = crashk_res.start & PAGE_MASK;
> > > > +	end = PAGE_ALIGN(crashk_res.end);
> > > > +
> > > > +	aligned_start = ALIGN_DOWN(crashk_res.start, PUD_SIZE);
> > > > +	aligned_end = ALIGN(end, PUD_SIZE);
> > > > +
> > > > +	/* Clear PUDs containing crash kernel memory */
> > > > +	unmap_hotplug_range(__phys_to_virt(aligned_start),
> > > > +			    __phys_to_virt(aligned_end), false, NULL);
> > > 
> > > What I don't understand is what happens if there's valid kernel data
> > > between aligned_start and crashk_res.start (or the other end of the
> > > range).
> > 
> > Data shouldn't go anywhere :)
> > 
> > There is 
> > 
> > +	/* map area from PUD start to start of crash kernel with large pages */
> > +	size = start - aligned_start;
> > +	__create_pgd_mapping(swapper_pg_dir, aligned_start,
> > +			     __phys_to_virt(aligned_start),
> > +			     size, PAGE_KERNEL, early_pgtable_alloc, 0);
> > 
> > and 
> > 
> > +	/* map area from end of crash kernel to PUD end with large pages */
> > +	size = aligned_end - end;
> > +	__create_pgd_mapping(swapper_pg_dir, end, __phys_to_virt(end),
> > +			     size, PAGE_KERNEL, early_pgtable_alloc, 0);
> > 
> > after the unmap, so after we tear down a part of a linear map we
> > immediately recreate it, just with a different page size.
> > 
> > This all happens before SMP, so there is no concurrency at that point.
> 
> That brief period of unmap worries me. The kernel text, data and stack
> are all in the vmalloc space but any other (memblock) allocation to this
> point may be in the unmapped range before and after the crashkernel
> reservation. The interrupts are off, so I think the only allocation and
> potential access that may go in this range is the page table itself. But
> it looks fragile to me.

I agree there are chances there will be an allocation from the unmapped
range. 

We can make sure this won't happen, though. We can cap the memblock
allocations with memblock_set_current_limit(aligned_end) or
memblock_reserve(algined_start, aligned_end) until the mappings are
restored. 

> -- 
> Catalin

-- 
Sincerely yours,
Mike.

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