Re: collision between ZONE_MOVABLE and memblock allocations

[Michal Hocko <mhocko@suse.com>]

On Wed 19-07-23 16:48:21, Ross Zwisler wrote:
> On Wed, Jul 19, 2023 at 08:14:48AM +0200, Michal Hocko wrote:
> > On Tue 18-07-23 16:01:06, Ross Zwisler wrote:
> > [...]
> > > I do think that we need to fix this collision between ZONE_MOVABLE and memmap
> > > allocations, because this issue essentially makes the movablecore= kernel
> > > command line parameter useless in many cases, as the ZONE_MOVABLE region it
> > > creates will often actually be unmovable.
> > 
> > movablecore is kinda hack and I would be more inclined to get rid of it
> > rather than build more into it. Could you be more specific about your
> > use case?
> 
> The problem that I'm trying to solve is that I'd like to be able to get kernel
> core dumps off machines (chromebooks) so that we can debug crashes.  Because
> the memory used by the crash kernel ("crashkernel=" kernel command line
> option) is consumed the entire time the machine is booted, there is a strong
> motivation to keep the crash kernel as small and as simple as possible.  To
> this end I'm trying to get away without SSD drivers, not having to worry about
> encryption on the SSDs, etc.
> 
> So, the rough plan right now is:
> 
> 1) During boot set aside some memory that won't contain kernel allocations.
> I'm trying to do this now with ZONE_MOVABLE, but I'm open to better ways.
> 
> We set aside memory for a crash kernel & arm it so that the ZONE_MOVABLE
> region (or whatever non-kernel region) will be set aside as PMEM in the crash
> kernel.  This is done with the memmap=nn[KMG]!ss[KMG] kernel command line
> parameter passed to the crash kernel.
> 
> So, in my sample 4G VM system, I see:
> 
>   # lsmem --split ZONES --output-all
>   RANGE                                  SIZE  STATE REMOVABLE BLOCK NODE   ZONES
>   0x0000000000000000-0x0000000007ffffff  128M online       yes     0    0    None
>   0x0000000008000000-0x00000000bfffffff  2.9G online       yes  1-23    0   DMA32
>   0x0000000100000000-0x000000012fffffff  768M online       yes 32-37    0  Normal
>   0x0000000130000000-0x000000013fffffff  256M online       yes 38-39    0 Movable
>   
>   Memory block size:       128M
>   Total online memory:       4G
>   Total offline memory:      0B
> 
> so I'll pass "memmap=256M!0x130000000" to the crash kernel.
> 
> 2) When we hit a kernel crash, we know (hope?) that the PMEM region we've set
> aside only contains user data, which we don't want to store anyway.  We make a
> filesystem in there, and create a kernel crash dump using 'makedumpfile':
> 
>   mkfs.ext4 /dev/pmem0
>   mount /dev/pmem0 /mnt
>   makedumpfile -c -d 31 /proc/vmcore /mnt/kdump
> 
> We then set up the next full kernel boot to also have this same PMEM region,
> using the same memmap kernel parameter.  We reboot back into a full kernel.

Btw. How do you ensure that the address range doesn't get reinitialized
by POST? Do you rely on kexec boot here?
-- 
Michal Hocko
SUSE Labs