On 11.09.25 19:56, Luck, Tony wrote:
On Thu, Sep 11, 2025 at 10:46:10AM +0200, David Hildenbrand wrote:
On 10.09.25 18:15, Kyle Meyer wrote:
Soft offlining a HugeTLB page reduces the available HugeTLB page pool.
Since HugeTLB pages are preallocated, reducing the available HugeTLB
page pool can cause allocation failures.
/proc/sys/vm/enable_soft_offline provides a sysctl interface to
disable/enable soft offline:
0 - Soft offline is disabled.
1 - Soft offline is enabled.
The current sysctl interface does not distinguish between HugeTLB pages
and other page types.
Disable soft offline for HugeTLB pages by default (1) and extend the
sysctl interface to preserve existing behavior (2):
0 - Soft offline is disabled.
1 - Soft offline is enabled (excluding HugeTLB pages).
2 - Soft offline is enabled (including HugeTLB pages).
Update documentation for the sysctl interface, reference the sysctl
interface in the sysfs ABI documentation, and update HugeTLB soft
offline selftests.
I'm sure you spotted that the documentation for
"/sys/devices/system/memory/soft_offline_pag" resides under "testing".
But that is only one of several places in the kernel that
feed into the page offline code.
Right, I can see one more call to soft_offline_page() from
arch/parisc/kernel/pdt.c.
And there is memory_failure_work_func() that I missed.
So agreed that this goes beyond testing.
It caught my attention because you ended up modifying documentation
residing in Documentation/ABI/testing/sysfs-memory-page-offline.
Reading 56374430c5dfc that Kyle pointed out is gets clearer.
So the patch motivation/idea makes sense to me.
I'll note two things:
(1) The interface design is not really extensible. Imagine if we want to
exclude yet another page type.
Can we maybe add a second interface that defines a filter for types?
Alternatively, you could use all the remaining flags as such a filter.
0 - Soft offline is completely disabled.
1 - Soft offline is enabled except for manually disabled types.
Filter
2 - disable hugetlb.
So value 3 would give you "enable all except hugetlb" etc.
We could add in the future
4 - disable guest_memfd (just some random example)
Then you
2) Changing the semantics of the value "1"
IIUC, you are changing the semantics of value "1". It used to mean
"SOFT_OFFLINE_ENABLED" now it is "SOFT_OFFLINE_ENABLED_SKIP_HUGETLB",
which is a change in behavior.
If that is the case, I don't think that's okay.
2) I am not sure about changing the default. That should be an admin/
distro decision.
--
Cheers
David / dhildenb
