Re: [PATCH V2] x86/kexec: do not update E820 kexec table for setup_data

[Jiri Bohac]

Hi,

On Thu, Mar 21, 2024 at 05:23:20PM +0800, Dave Young wrote:
> crashkernel reservation failed on a Thinkpad t440s laptop recently. 
> Actually the memblock reservation succeeded, but later insert_resource()
> failed.
> 
> Test steps:
> kexec load -> /* make sure add crashkernel param eg. crashkernel=160M */
>     kexec reboot -> 
>         dmesg|grep "crashkernel reserved";
>             crashkernel memory range like below reserved successfully:
>             0x00000000d0000000 - 0x00000000da000000
>         But no such "Crash kernel" region in /proc/iomem
> 
> The background story is like below:
> 
> Currently E820 code reserves setup_data regions for both the current
> kernel and the kexec kernel, and it inserts them into the resources list.
> Before the kexec kernel reboots nobody passes the old setup_data, and
> kexec only passes fresh SETUP_EFI and SETUP_IMA if needed.  Thus the old
> setup data memory is not used at all.
> 
> Due to old kernel updates the kexec e820 table as well so kexec kernel
> sees them as E820_TYPE_RESERVED_KERN regions, and later the old setup_data
> regions are inserted into resources list in the kexec kernel by
> e820__reserve_resources().
> 
> Note, due to no setup_data is passed in for those old regions they are not
> early reserved (by function early_reserve_memory), and the crashkernel
> memblock reservation will just treat them as usable memory and it could
> reserve the crashkernel region which overlaps with the old setup_data
> regions. And just like the bug I noticed here, kdump insert_resource
> failed because e820__reserve_resources has added the overlapped chunks
> in /proc/iomem already.

wouldn't this be caused by
4a693ce65b186fddc1a73621bd6f941e6e3eca21 ("kdump: defer the
insertion of crashkernel resources")?

Before that the crashkernel resources were inserted from
arch_reserve_crashkernel() which is called before
e820__reserve_resources().

The semantics of E820_TYPE_RESERVED_KERN wrt kexec quite
inconsistent. It's treated as E820_TYPE_RAM by
e820__memblock_setup() and e820_type_to_iomem_type().

The problem we're seeing here is the result of the former.
e820__memblock_setup() will add the E820_TYPE_RESERVED_KERN
region to the memblock, merging with the neighbouring memblocks,
allowing crashkernel region to span across the originally
reserved area.

e820_type_to_iomem_type() treating E820_TYPE_RESERVED_KERN as
E820_TYPE_RAM will make the E820_TYPE_RESERVED_KERN appear as
system ram in /proc/iomem. If the old kexec_load (not
kexec_file_load) syscall is used, the userspace kexec utility
will construct the e820 table based on the contents of
/proc/iomem and the kexec kernel will see the
E820_TYPE_RESERVED_KERN range as E820_TYPE_RAM.  When
kexec_file_load is used the E820_TYPE_RESERVED_KERN type is
propagated to the kexec kernel by bzImage64_load() /
setup_e820_entries().


> Index: linux/arch/x86/kernel/e820.c
> ===================================================================
> --- linux.orig/arch/x86/kernel/e820.c
> +++ linux/arch/x86/kernel/e820.c
> @@ -1015,16 +1015,6 @@ void __init e820__reserve_setup_data(voi
>  		pa_next = data->next;
>  
>  		e820__range_update(pa_data, sizeof(*data)+data->len, E820_TYPE_RAM, E820_TYPE_RESERVED_KERN);
> -
> -		/*
> -		 * SETUP_EFI and SETUP_IMA are supplied by kexec and do not need
> -		 * to be reserved.
> -		 */
> -		if (data->type != SETUP_EFI && data->type != SETUP_IMA)
> -			e820__range_update_kexec(pa_data,
> -						 sizeof(*data) + data->len,
> -						 E820_TYPE_RAM, E820_TYPE_RESERVED_KERN);
> -

Your tree is missing this recent commit:
7fd817c906503b6813ea3b41f5fdf4192449a707 ("x86/e820: Don't
reserve SETUP_RNG_SEED in e820").

Wouldn't this fix [/paper over] your problem as well? I.e., isn't
SETUP_RNG_SEED the setup_data item that's causing your problem?

Regards,

-- 
Jiri Bohac <jbohac@xxxxxxx>
SUSE Labs, Prague, Czechia