Re: collision between ZONE_MOVABLE and memblock allocations

[David Hildenbrand <david@redhat.com>]

On 26.07.23 10:44, David Hildenbrand wrote:
> On 20.07.23 00:48, 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.
> 
> Okay, so you intend to keep the crashkernel area as small as possible.
> 
>>
>> 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.
> 
> I raised that in different context already, but such assumptions are not
> 100% future proof IMHO. For example, we might at one point be able to
> make user page tables movable and place them on there.
> 
> But yes, most kernel data structures (which you care about) will
> probably never be movable and never end up on these regions.
> 
>> 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.
>>
>> 3) The next full kernel will be a normal boot with a full networking stack,
>> SSD drivers, disk encryption, etc.  We mount up our PMEM filesystem, pull out
>> the kdump and either store it somewhere persistent or upload it somewhere.  We
>> can then unmount the PMEM and reconfigure it back to system ram so that the
>> live system isn't missing memory.
>>
>>     ndctl create-namespace --reconfig=namespace0.0 -m devdax -f
>>     daxctl reconfigure-device --mode=system-ram dax0.0
>>
>> This is the flow I'm trying to support, and have mostly working in a VM,
>> except up until now makedumpfile would crash because all the memblock
>> structures it needed were in the PMEM area that I had just wiped out by making
>> a new filesystem. :)
> 
> 
> Thinking out loud (and remembering that some architectures relocate the
> crashkernel during kexec, if I am not wrong), maybe the following would
> also work and make your setup eventually easier:
> 
> 1) Don't reserve a crashkernel area in the traditional way, instead
> reserve that area using CMA. It can be used for MOVABLE allocations.
> 
> 2) Let kexec load the crashkernel+initrd into ordinary memory only
> (consuming as much as you would need there).

Oh, I realized that one might be able to place the kernel+initrd 
directly in the area by allocating via CMA.

-- 
Cheers,

David / dhildenb