On 30.05.25 11:52, Baolin Wang wrote:
On 2025/5/30 17:16, David Hildenbrand wrote:
On 30.05.25 11:10, David Hildenbrand wrote:
On 30.05.25 10:59, Ryan Roberts wrote:
On 30/05/2025 09:44, David Hildenbrand wrote:
On 30.05.25 10:04, Ryan Roberts wrote:
On 29/05/2025 09:23, Baolin Wang wrote:
As we discussed in the previous thread [1], the MADV_COLLAPSE will
ignore
the system-wide anon/shmem THP sysfs settings, which means that
even though
we have disabled the anon/shmem THP configuration, MADV_COLLAPSE
will still
attempt to collapse into a anon/shmem THP. This violates the rule
we have
agreed upon: never means never. This patch set will address this
issue.
This is a drive-by comment from me without having the previous
context, but...
Surely MADV_COLLAPSE *should* ignore the THP sysfs settings? It's a
deliberate
user-initiated, synchonous request to use huge pages for a range of
memory.
There is nothing *transparent* about it, it just happens to be
implemented using
the same logic that THP uses.
I always thought this was a deliberate design decision.
If the admin said "never", then why should a user be able to
overwrite that?
Well my interpretation would be that the admin is saying never
*transparently*
give anyone any hugepages; on balance it does more harm than good for my
workloads. The toggle is called transparent_hugepage/enabled, after all.
I'd say it's "enabling transparent huge pages" not "transparently
enabling huge pages". After all, these things are ... transparent huge
pages.
But yeah, it's confusing.
Whereas MADV_COLLAPSE is deliberately applied to a specific region at an
opportune moment in time, presumably because the user knows that the
region
*will* benefit and because that point in the execution is not
sensitive to latency.
Not sure if MADV_HUGEPAGE is really *that* different.
I see them as logically separate.
The design decision I recall is that if VM_NOHUGEPAGE is set, we'll
ignore that.
Because that was set by the app itself (MADV_NOHUEPAGE).
IIUC, MADV_COLLAPSE does not ignore the VM_NOHUGEPAGE setting, if we set
VM_NOHUGEPAGE, then MADV_COLLAPSE will not be allowed to collapse a THP.
See:
__thp_vma_allowable_orders() ---> vma_thp_disabled()
Interesting, maybe I misremember things.
Maybe because process_madvise() could try MADV_COLLAPSE on a different
process. And if that process as VM_NOHUGEPAGE set, it could be problematic.
--
Cheers,
David / dhildenb
