Re: [PATCH] x86/mm: only allow memmap=XX!YY over existing RAM

[Boaz Harrosh]

On 06/20/2016 10:33 AM, Yigal Korman wrote:
> Before this patch, passing a range that is beyond the physical memory
> range will succeed, the user will see a /dev/pmem0 and will be able to
> access it. Reads will always return 0 and writes will be silently
> ignored.
> 
> I've gotten more than one bug report about mkfs.{xfs,ext4} or nvml
> failing that were eventually tracked down to be wrong values passed to
> memmap.
> 
> This patch prevents the above issue by instead of adding a new memory
> range, only update a RAM memory range with the PRAM type. This way,
> passing the wrong memmap will either not give you a pmem at all or give
> you a smaller one that actually has RAM behind it.
> 
> And if someone still needs to fake a pmem that doesn't have RAM behind
> it, they can simply do memmap=XX@YY,XX!YY.
> 

We are running with this patch for a while in the lab and it does
solve the problem above with no maleffects so:

Tested-by: Boaz Harrosh <boaz@xxxxxxxxxxxxx>

> Signed-off-by: Yigal Korman <yigal@xxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/e820.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> index 621b501..4bd4207 100644
> --- a/arch/x86/kernel/e820.c
> +++ b/arch/x86/kernel/e820.c
> @@ -878,7 +878,7 @@ static int __init parse_memmap_one(char *p)
>  		e820_add_region(start_at, mem_size, E820_RESERVED);
>  	} else if (*p == '!') {
>  		start_at = memparse(p+1, &p);
> -		e820_add_region(start_at, mem_size, E820_PRAM);
> +		e820_update_range(start_at, mem_size, E820_RAM, E820_PRAM);
>  	} else
>  		e820_remove_range(mem_size, ULLONG_MAX - mem_size, E820_RAM, 1);
>  
>