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Date:   Fri, 31 Aug 2018 13:33:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Feng Tang <feng.tang@...el.com>
cc:     Michal Hocko <mhocko@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Ingo Molnar <mingo@...e.hu>,
        "H . Peter Anvin" <hpa@...ux.intel.com>,
        Yinghai Lu <yinghai@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86, mm: Reserver some memory for bootmem allocator for
 NO_BOOTMEM

On Fri, 31 Aug 2018, Feng Tang wrote:
> On Thu, Aug 30, 2018 at 03:25:42PM +0200, Thomas Gleixner wrote:
> This panic happens as the earlycon's fixmap address has no
> pmd/pte ready, and __set_fixmap will try to allocate memory to
> setup the page table, and trigger panic due to no memory.
> 
> x86 kernel actually prepares the page table for fixmap in head_64.S:
> 
> 	NEXT_PAGE(level2_fixmap_pgt)
> 		.fill	506,8,0
> 		.quad	level1_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE_NOENC
> 		/* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
> 		.fill	5,8,0
> 
> and it expects the fixmap address is in [-12M, -10M] range, but
> current code in fixmap.h will break the expectation when
> X86_VSYSCALL_EMULATION=n
> 
> 	#ifdef CONFIG_X86_VSYSCALL_EMULATION
> 		VSYSCALL_PAGE = (FIXADDR_TOP - VSYSCALL_ADDR) >> PAGE_SHIFT,
> 	#endif
> 
> So removing the "#ifdef" will make the fixmap address space stable in
> [-12M, -10M] and fix the issue.

Why on earth are you not fixing the damned PTE setup which is the obvious
and correct thing to do?

Thanks,

	tglx


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