On 04/09/26 21:52, Pratyush Yadav wrote:
On Fri, Sep 04 2026, Sourabh Jain wrote:
On 02/09/26 16:04, Pratyush Yadav wrote:
On Sun, Aug 23 2026, Sourabh Jain wrote:
On 21/08/26 17:26, Pratyush Yadav wrote:
On Fri, Aug 21 2026, Sourabh Jain wrote:
[...]
I agree that this is one way to work around the low-memory reservation problem.
However, there are a few things that come into play here:
1. On powerpc, the crashkernel reservation can go up to 64 GB for kdump. With
the
current default scratch memory reservation policy, this could result in
reserving
up to 256 GB of scratch memory: 200% for the high-memory reservation and
another
200% for per-node memory.
That calculation looks off. It _should_ be 200% once not twice. So 128
GB total. If the allocation came out via the global area, it should
_only_ be accounted to the global scratch size. Similarly, only the
allocations made specifically on that node should be counted for the
per-node scratch size.
For example, if a system has only one node and 64 GB is allocated from
that node before the kernel starts calculating the per-node and global
allocations for scratch memory, wouldn't the per-node allocation also be 64 GB?
If so, wouldn't that result in 200% of 64 GB being allocated for the global
area and another 200% of 64 GB for the per-node area, resulting in 256 GB
of total scratch memory allocation? Or am I missing something here?
It shouldn't. If the 64 GB of allocation was done with NUMA_NO_NODE, and
it _happened_ to land on node X, it should not be counted for per-node
sizing. It should count towards the global pool. Only allocations that
were explicitly requested with node X should be count for that node's
scratch size.
So on a one node system where 64G of memory is allocated with
NUMA_NO_NODE and 8G is allocated with node X, we should get 128G of
global scratch and 16G of per-node scratch, giving us a total of 144G.
That makes sense. I was just going by the reservation that was made. As
you mentioned, an allocation made on a specific node can be
differentiated from a general allocation because users pass the NID for
a specific NUMA allocation and |NUMA_NO_NODE|for a general allocation.
Thanks for the clarification!
I took a quick look and it looks like the problem might be that the
calculation for global scratch includes _all_ nodes in it. See
memblock_reserved_kern_size():
for_each_reserved_mem_region(r) {
...
if (nid == memblock_get_region_node(r) || !numa_valid_node(nid))
if (r->flags & MEMBLOCK_RSRV_KERN)
total += size;
}
And for global scratch we pass nid as NUMA_NO_NODE.
For KHO we could just drop the || !numa_valid_node(), but
memblock_estimated_nr_free_pages() seems to depend on that behaviour. It
wants to get _all_ allocations across all nodes. KHO only wants
allocations explicitly made with NUMA_NO_NODE.
Yes the above explanation seems correct to me. !numa_valid_node() is
problem in KHO scratch memory reservation context.
But disclaimer: all this is from reading the code for maybe 15 minutes.
I didn't run anything and might be missing something. So please
double-check what I am saying.
Not sure how to fix this. Since memblock_estimated_nr_free_pages() needs
all the reservations anyway, perhaps open code a simple counting loop
there? And the drop the || !numa_valid_node() from
memblock_reserved_kern_size().
But yeah, it would be much appreciated if you'd care to fix this.
I will send a fix for this.
The fix should be a separete patch, since it fixes problems on all
platforms, and not just PowerPC.
Yes the fix shouldn't be part of this series...
But I have also noticed this problem on some of the systems Google has.
Which makes me wonder if scratch_size_update() is broken and
over-calculating. I have this on my TODO list and have been meaning to
look into it, but other things keep intervening.
If you are interested, feel free to take it off my hands.
Yes, I can take this up and propose patches to make crashkernel and
scratch reservations work together.
Based on my current testing, a Linux partition (powerpc) with 16 CPUs and 30 GB
of RAM needs only 16 MB of scratch memory in the low-memory area when
crashkernel=xxM is not specified.
16 MB is not much. I am also trying to get a larger Linux partition with 1000+
CPUs to get a better idea of the limits for low-memory reservations.
BTW, do you know the rationale behind the 200% value?
I couldn't find any explanation for it in the commit message of
3dc92c311498c ("kexec: add Kexec HandOver (KHO) generation helpers")
We need to ask Alex (or maybe Mike?; I forget who added this).
But if I were to guess, I don't think there is much science involved
behind the number. Since the scratch lives across all kexecs, it needs
to be large enough in case the next kernel uses more memory. 200% sounds
"large enough".
OK..
For fadump, which is the powerpc-specific memory dump capture mechanism,
the
crashkernel
reservation can go up to 180 GB. In this case, we could end up reserving up
to 720 GB of
scratch memory, which is too much. I agree that users can tune this, but I
think the
default scale should be more reasonable for powerpc.
Once we fix scratch_size_update() to actually use 200% and not 400%,
perhaps that alone will be enough? If not, we can discuss reducing the
default scratch scale to maybe 150%. But I'd rather do it for all
platforms if we do it at all, because this problem doesn't seem specific
to PowerPC.
Yes, it makes sense to have a general fix that works for all architectures.
BTW, I was able to reproduce this issue on x86 as well. Please have a
look at this:
https://lore.kernel.org/all/[email protected]/
I have also suggested an approach to handle this issue which is similar how you
handle
huge pages. Please share your thoughts on it.
I missed this.
We can exclude HugeTLB pages from scratch accounting because the series
updates HugeTLB to use a new routine called memblock_alloc_hugetlb() to
allocate pages. This special allocator makes sure the pages are
_outside_ of scratch even if scratch-only mode is used. Since HugeTLB
pages come outside of scratch, they don't get counted in scratch sizing.
We need to do this for HugeTLB mainly because we want to preserve
HugeTLB pages in the future, and pages from scratch can't be preserved.
We could perhaps do so for crash as well, but I need to think more about
this.
But at first glance, if you need to allocate crash super early, perhaps
memblock won't be able to cope. Because allocating crash outside of
scratch would depend on kho_extend_scratch() and that needs
memblock_allow_resize() to be called to be able to cope with multiple
memory regions.
I just saw your patches to handle HugeTLB separately. I was thinking of
handling the crashkernel reservation in a similar way.
Let me first understand the HugeTLB handling in KHO completely, and then
I can propose a similar approach for the crashkernel reservation. This
should allow the crashkernel and scratch reservations to coexist in most
cases.
[...]
Could you please elaborate a bit on what makes the ordering tricky and what the
main tradeoffs are between crash reservations and KHO? It would help me better
understand the concerns here.
The problem today is that kho_preserved_memory_reserve() (called by
kho_mem_retrieve()) does a memblock_reserve() for each preserved folio.
So if you have a lot of order-0 (or, 4k) folios, you end up with a lot
of reservations in memblock. The large number of reservations can slow
down later memblock operations like allocations too since memblock might
have to walk through a lot of ranges to find free memory.
We kind of work around this problem by calling kho_mem_retrieve() as
pretty much the last thing in the MM init. So all allocations prior to
this have already been fulfilled from scratch without any of the
reservations added, so it should be pretty fast. You only take the
performance hit at the end, where the only thing left is to release
pages to buddy.
Ah, okay, that makes sense. Thanks for the clarification.
Even then, the memblock reservations can get pretty damn slow. In some
of my testing with under-load systems, preserving a 2G memfd with 4k
pages can go over **5 minutes** in only kho_mem_retrieve() if the folios
of the memfd are fragmented enough. Plus there is the memory overhead of
the regions in memblock.reserved.
5 minutes in kho_mem_retrieve(), which is primarily marking a bunch
of memory as reserved using memblock, seems like quite a lot. If you
have the test case handy somewhere, I would be interested in trying it
myself, just to get a better feel for the issue.
I do, but unfortunately based on downstream code so it is neither useful
to you nor something I can share I think.
But your friendly neighbourhood LLM can help here. Ask it to preserve
you a memfd but fragment/shatter buddy blocks first. That's pretty much
how I wrote my test.
Sure I will try it and share my experience.
But also see [0] which fixes the problem. Maybe Tarun (+Cc) has a test
based on upstream that he can share?
[0] https://lore.kernel.org/kexec/[email protected]/
Thanks for the Cc. I will try the above patch also.
Regardless, I understand the concern now. From my perspective also, the
current ordering of crashkernel and scratch memory reservations seems
reasonable, because crashkernel is not as flexible as scratch reservation
atleast on powerpc.
On powerpc, the crashkernel offset is determined first, and the
corresponding memory region is reserved. To make sure that no
other reservation falls within the crashkernel region, the crashkernel
reservation is one of the first reservations we make on powerpc.
If we change this ordering, there is a possibility that a scratch
reservation could end up in a region where the crashkernel is supposed
to be placed. That would lead to crashkernel reservation failure.
Also, reserving scratch memory at a location where the crashkernel
cannot be placed could be problematic. Each architecture has its own
constraints on where the crashkernel can be placed, so the available
memory for scratch reservation may need to account for those constraints.
Which I think too much to take care off...
That's a real problem. But I am hoping the restrictions are something
along the lines of "crash kernel must be in lowmem", so the lowmem
scratch already solves that problem?
Yes, for different reasons, the architecture may need to load the
vmlinux kexec segment in low memory. Alternatively, there may need to
be some crashkernel reservation in low memory to satisfy the memory
requirements of components that can only allocate memory from low
memory.
I will also explore how KHO handles reserved memory regions.
For example, let’s say the firmware marks 10 MB as reserved and exports
that region as reserved in the DTB. If I am not mistaken, while parsing the
FDT, the kernel calls memblock_reserve() for these regions. PowerPC has
some such regions, like RTAS and OPAL. Generally, these regions should
persist across KHO.
So I will explore:
- How these regions are handled by KHO
- How they affect the scratch kernel reservation, specially in lowmem
- Sourabh Jain
And, of course, moving kho_mem_retrieve()earlier during boot would
also mean taking the performance hit you mentioned earlier.
So let's keep the current ordering and find a way to make both
reservations work with it: reserve the crashkernel first, and then
reserve the scratch memory.
[...]
