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

[Feng Tang]

Hi Thomas,

On Fri, Aug 31, 2018 at 09:36:59PM +0800, Feng Tang wrote:
> 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:

Could you review this patch, as at this time window there is no usable memory
block or other memory allocators that I know, so I follow the exisitng static
fixmap page table code and add one more.

Thanks,
Feng

> 
> ---
> 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
>