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

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Message-ID: <YsVeKPzaO0SJdwFW@arm.com>
Date:   Wed, 6 Jul 2022 11:04:24 +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 11:45:40PM +0300, Mike Rapoport wrote:
> 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. 

We can reserve the region just before unmapping to avoid new allocations
for the page tables but we can't do much about pages already allocated
prior to calling remap_crashkernel().

-- 
Catalin