linux-kernel - Re: [PATCH] x86, mm: Reserver some memory for bootmem allocator for NO

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Date:   Fri, 31 Aug 2018 21:36:59 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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, Aug 31, 2018 at 01:33:05PM +0200, Thomas Gleixner wrote:
> 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?

Any sugestion? I can only have patch like this:

---
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 15ebc2fc166e..8cdb27ccc3a3 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -446,11 +446,15 @@ NEXT_PAGE(level2_kernel_pgt)
 
 NEXT_PAGE(level2_fixmap_pgt)
 	.fill	506,8,0
-	.quad	level1_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE_NOENC
+	.quad	level1_fixmap_pgt0 - __START_KERNEL_map + _PAGE_TABLE_NOENC
+	.quad	level1_fixmap_pgt1 - __START_KERNEL_map + _PAGE_TABLE_NOENC
 	/* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
-	.fill	5,8,0
+	.fill	4,8,0
 
-NEXT_PAGE(level1_fixmap_pgt)
+NEXT_PAGE(level1_fixmap_pgt0)
+	.fill	512,8,0
+
+NEXT_PAGE(level1_fixmap_pgt1)
 	.fill	512,8,0
 
 #undef PMDS


Thanks,
Feng