Re: [PATCH] x86/mm: Fix boot with some memory above MAXMEM

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Tony Luck <tony.luck@intel.com>,
	x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Dave Hansen <dave.hansen@intel.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH] x86/mm: Fix boot with some memory above MAXMEM
Date: Mon, 25 May 2020 07:49:02 +0300	[thread overview]
Message-ID: <20200525044902.rsb46bxu5hdsqglt@box> (raw)
In-Reply-To: <20200511191721.1416-1-kirill.shutemov@linux.intel.com>

On Mon, May 11, 2020 at 10:17:21PM +0300, Kirill A. Shutemov wrote:
> A 5-level paging capable machine can have memory above 46-bit in the
> physical address space. This memory is only addressable in the 5-level
> paging mode: we don't have enough virtual address space to create direct
> mapping for such memory in the 4-level paging mode.
> 
> Currently, we fail boot completely: NULL pointer dereference in
> subsection_map_init().
> 
> Skip creating a memblock for such memory instead and notify user that
> some memory is not addressable.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reviewed-by: Dave Hansen <dave.hansen@intel.com>
> Cc: stable@vger.kernel.org # v4.14
> ---

Gentle ping.

It's not urgent, but it's a bug fix. Please consider applying.

> Tested with a hacked QEMU: https://gist.github.com/kiryl/d45eb54110944ff95e544972d8bdac1d
> 
> ---
>  arch/x86/kernel/e820.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> index c5399e80c59c..d320d37d0f95 100644
> --- a/arch/x86/kernel/e820.c
> +++ b/arch/x86/kernel/e820.c
> @@ -1280,8 +1280,8 @@ void __init e820__memory_setup(void)
>  
>  void __init e820__memblock_setup(void)
>  {
> +	u64 size, end, not_addressable = 0;
>  	int i;
> -	u64 end;
>  
>  	/*
>  	 * The bootstrap memblock region count maximum is 128 entries
> @@ -1307,7 +1307,22 @@ void __init e820__memblock_setup(void)
>  		if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
>  			continue;
>  
> -		memblock_add(entry->addr, entry->size);
> +		if (entry->addr >= MAXMEM) {
> +			not_addressable += entry->size;
> +			continue;
> +		}
> +
> +		end = min_t(u64, end, MAXMEM - 1);
> +		size = end - entry->addr;
> +		not_addressable += entry->size - size;
> +		memblock_add(entry->addr, size);
> +	}
> +
> +	if (not_addressable) {
> +		pr_err("%lldGB of physical memory is not addressable in the paging mode\n",
> +		       not_addressable >> 30);
> +		if (!pgtable_l5_enabled())
> +			pr_err("Consider enabling 5-level paging\n");
>  	}
>  
>  	/* Throw away partial pages: */
> -- 
> 2.26.2
> 
> 

-- 
 Kirill A. Shutemov