On 21/12/25 11:29, Ritesh Harjani (IBM) wrote:
Hi Sourabh,
Sourabh Jain <[email protected]> writes:
Skip processing hugepage kernel arguments (hugepagesz, hugepages, and
default_hugepagesz) when hugepages are not supported by the
architecture.
Some architectures may need to disable hugepages based on conditions
discovered during kernel boot. The hugepages_supported() helper allows
architecture code to advertise whether hugepages are supported.
Currently, normal hugepage allocation is guarded by
hugepages_supported(), but gigantic hugepages are allocated regardless
of this check. This causes problems on powerpc for fadump (firmware-
assisted dump).
In the fadump (firmware-assisted dump) scenario, a production kernel
crash causes the system to boot into a special kernel whose sole
purpose is to collect the memory dump and reboot. Features such as
hugepages are not required in this environment and should be
disabled.
For example, when the fadump kernel boots with the following kernel
arguments:
default_hugepagesz=1GB hugepagesz=1GB hugepages=200
Before this patch, the kernel prints the following logs:
HugeTLB: allocating 200 of page size 1.00 GiB failed. Only allocated 58
hugepages.
HugeTLB support is disabled!
HugeTLB: huge pages not supported, ignoring associated command-line parameters
hugetlbfs: disabling because there are no supported hugepage sizes
Even though the logs state that HugeTLB support is disabled, gigantic
hugepages are still allocated. This causes the fadump kernel to run out
of memory during boot.
After this patch is applied, the kernel prints the following logs for
the same command line:
HugeTLB: hugepages unsupported, ignoring default_hugepagesz=1GB cmdline
HugeTLB: hugepages unsupported, ignoring hugepagesz=1GB cmdline
HugeTLB: hugepages unsupported, ignoring hugepages=200 cmdline
HugeTLB support is disabled!
hugetlbfs: disabling because there are no supported hugepage sizes
To fix the issue, gigantic hugepage allocation should be guarded by
hugepages_supported().
Previously, two approaches were proposed to bring gigantic hugepage
allocation under hugepages_supported():
[1] Check hugepages_supported() in the generic code before allocating
gigantic hugepages
[2] Make arch_hugetlb_valid_size() return false for all hugetlb sizes
Approach [2] has two minor issues:
1. It prints misleading logs about invalid hugepage sizes
2. The kernel still processes hugepage kernel arguments unnecessarily
To control gigantic hugepage allocation, skip processing hugepage kernel
arguments (default_hugepagesz, hugepagesz and hugepages) when
hugepages_supported() returns false.
Link:
https://lore.kernel.org/all/[email protected]/
[1]
Link:
https://lore.kernel.org/all/[email protected]/
[2]
Fixes: c2833a5bf75b ("hugetlbfs: fix changes to command line processing")
I appreciate our proactiveness to respond quickly on mailing list, but I
suggest we give enough time to folks before sending the next version
please ;).
I agree that I posted the v6 too quickly. I will avoid that in future.
Your email from last night [1] says that we will use this fixes tag but
you haven't even given us 24hrs to respond to that email thread :). Now
we've sent this v6, with Acked-by of David and Reviewed-by of mine,
which seems like everything was agreed upon, but that isn't the case
actually.
Yes, you are right. I should have waited until the discussion about the
Fixes tag was finished.
Thanks for pointing out things Ritesh.
- Sourabh Jain
