On 2025-02-28 00:17, Linus Torvalds wrote:
On Thu, 27 Feb 2025 at 19:03, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
I'd be fine with SKSM replacing KSM entirely. However, I don't
think we should try to re-implement the existing KSM userspace ABIs
over SKSM.
No, absolutely. The only point (for me) for your new synchronous one
would be if it replaced the kernel thread async scanning, which would
make the old user space interface basically pointless.
But I don't actually know who uses KSM right now. My reaction really
comes from a "it's not nice code in the kernel", not from any actual
knowledge of the users.
Maybe it works really well in some cloud VM environment, and we're
stuck with it forever.
For the VM use-case, I wonder if we could just add a userfaultfd
"COW" event that would notify userspace when a COW happens ?
This would allow userspace to replace ksmd by tracking the age of
those anonymous pages, and issue madvise MADV_MERGE on them to
write-protect+merge them when it is deemed useful.
With both a new userfaultfd COW event and madvise MADV_MERGE,
is there anything else that is fundamentally missing to move
all the scanning complexity of KSM to userspace for the VM
deduplication use-case ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
