Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 5 Jul 2022 18:57:53 +0300
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
Message-ID: <YsRfgX7FFZLxQU50@kernel.org>
References: <2ae1cae0-ee26-aa59-7ed9-231d67194dce@linux.alibaba.com>
 <20220704142313.GE31684@willie-the-truck>
 <6977c692-78ca-5a67-773e-0389c85f2650@linux.alibaba.com>
 <20220704163815.GA32177@willie-the-truck>
 <CAMj1kXEvY5QXOUrXZ7rBp9As=65uTTFRSSq+FPt-n4M2P-_VtQ@mail.gmail.com>
 <20220705095231.GB552@willie-the-truck>
 <5d044fdd-a61a-d60f-d294-89e17de37712@linux.alibaba.com>
 <20220705121115.GB1012@willie-the-truck>
 <YsRSajyMxahXe7ZS@kernel.org>
 <YsRZ8V8mQ+HM31D6@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <YsRZ8V8mQ+HM31D6@arm.com>
Precedence: bulk

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.

> -- 
> Catalin

-- 
Sincerely yours,
Mike.