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

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Message-ID: <YsRvPTORdvIwzShL@arm.com>
Date:   Tue, 5 Jul 2022 18:05:01 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        "guanghui.fgh" <guanghuifeng@...ux.alibaba.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        baolin.wang@...ux.alibaba.com, akpm@...ux-foundation.org,
        david@...hat.com, jianyong.wu@....com, james.morse@....com,
        quic_qiancai@...cinc.com, christophe.leroy@...roup.eu,
        jonathan@...ek.ca, mark.rutland@....com,
        thunder.leizhen@...wei.com, anshuman.khandual@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        geert+renesas@...der.be, linux-mm@...ck.org,
        yaohongbo@...ux.alibaba.com, alikernel-developer@...ux.alibaba.com
Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance
 degradation

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.

-- 
Catalin