Re: [PATCH] x86, mm: Reserver some memory for bootmem allocator for NO_BOOTMEM

[Feng Tang]

Hi Thomas,

On Mon, Sep 10, 2018 at 11:53:39AM +0200, Thomas Gleixner wrote:
> On Mon, 10 Sep 2018, Feng Tang wrote:
> > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > index e848a4811785..fc172551048a 100644
> > --- a/arch/x86/mm/pgtable.c
> > +++ b/arch/x86/mm/pgtable.c
> > @@ -637,6 +637,19 @@ void __native_set_fixmap(enum fixed_addresses idx, pte_t pte)
> >  {
> >  	unsigned long address = __fix_to_virt(idx);
> >  
> > +#ifdef CONFIG_X86_64
> > +	/*
> > +	 * In arch/x86/kernel/head_64.S, the static page table
> > +	 * has been setup for 4M space [-12M, -8M]
> > +	 *	0xFFFFFFFFFF400000 ~ 0xFFFFFFFFFF7FFFFF
> > +	 * Add a sanity check whether fixed_address crosses
> > +	 * the boundary.
> > +	 */
> > +	#define FIXMAP_STATIC_PGTABLE_START	0xFFFFFFFFFF400000
> 
> Errm what? This is beyond hillarious, really. What helps that if I remove
> that second FIXMAP entry in head_64.S?
> 
> The size of the fixmap is known at compile time and it is known when
> building head_64.S. So the obvious check is to check right there where the
> static pagetable is set up and where you know how many entries you set up
> whether it's covering the full size or not and err out there.

Thanks for the suggestion, and I have 2 patches: one adds a build warning,   
the other prepares fixmap page table on demand and doesn't need warning.

But I met a problem, that the "__end_of_permanent_fixed_addresses" is
defined in fixmap.h, which is protected by #ifndef __ASSEMBLY__, also
fixmap.h reference many other header file, which makes it harder to
extract the definition out. Any suggestion on this? thanks!

- Feng

patch1:
---
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 3a1d33b43c29..5863623890af 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -451,12 +451,20 @@ NEXT_PAGE(level2_fixmap_pgt)
 	.quad	level1_fixmap_pgt1 - __START_KERNEL_map + _PAGE_TABLE_NOENC
 	.fill	4,8,0
 
+/*
+ * Next are static page table for 4M fixmap space:
+ *	[-12M, -10M], [-10M, -8M]
+ */
 NEXT_PAGE(level1_fixmap_pgt)
 	.fill	512,8,0
 
 NEXT_PAGE(level1_fixmap_pgt1)
 	.fill	512,8,0
 
+#if __end_of_permanent_fixed_addresses > 1024
+.error "Total fixmap space exceeds the static page table capacity, please expand the page table!"
+#endif
+
 #undef PMDS
 
 	.data


Patch2:
---
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 15ebc2fc166e..2b98ce234686 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -24,6 +24,7 @@
 #include "../entry/calling.h"
 #include <asm/export.h>
 #include <asm/nospec-branch.h>
+#include <asm/fixmap.h>
 
 #ifdef CONFIG_PARAVIRT
 #include <asm/asm-offsets.h>
@@ -444,14 +445,21 @@ NEXT_PAGE(level2_kernel_pgt)
 	PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
 		KERNEL_IMAGE_SIZE/PMD_SIZE)
 
+#define FIXMAP_PMD_NUM	((__end_of_permanent_fixed_addresses + 511) / 512)
 NEXT_PAGE(level2_fixmap_pgt)
-	.fill	506,8,0
-	.quad	level1_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE_NOENC
+	.fill	(512 - 4 - FIXMAP_PMD_NUM),8,0
+	pgtno = 0
+	.rept (FIXMAP_PMD_NUM)
+	.quad level1_fixmap_pgt + (pgtno << PAGE_SHIFT) - __START_KERNEL_map  + _PAGE_TABLE_NOENC
+	pgtno = pgtno + 1
+	.endr
 	/* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
-	.fill	5,8,0
+	.fill	4,8,0
 
 NEXT_PAGE(level1_fixmap_pgt)
+	.rept (FIXMAP_PMD_NUM)
 	.fill	512,8,0
+	.endr
 
 #undef PMDS