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).
Hmm, ok. My instinct would have been the opposite; MADV_NOHUGEPAGE means "I
don't want the risk of latency spikes and memory bloat that THP can cause". Not
"ignore my explicit requests to MADV_COLLAPSE".
But if that descision was already taken and that's the current behavior then I
agree we have an inconsistency with respect to the sysfs control.
Perhaps we should be guided by real world usage - AIUI there is a cloud that
disables THP at system level today (Google?).
The use case I am aware of for disabling it for debugging purposes.
Saved us quite some headake in the past at customer sites for
troubleshooting + workarounds ...
Let's take a look at the man page:
MADV_COLLAPSE is independent of any sysfs (see sysfs(5)) setting
under /sys/kernel/mm/transparent_hugepage, both in terms of determining
THP eligibility, and allocation semantics.
I recall we discussed that it should ignore the max_ptes_none/swap/shared.
But "any" setting would include "enable" ...
It kind-of contradicts the linked
Documentation/admin-guide/mm/transhuge.rst, where we have this
*beautiful* comment
"Transparent Hugepage Support for anonymous memory can be entirely
disable (mostly for debugging purposes".
I mean, "entirely" is also pretty clear to me.
I would assume that the man page of MADV_COLLAPSE should have talked
about ignoring *khugepaged* toggles (max_ptes_none ...), at least that's
what I recall from the discussions back then.
--
Cheers,
David / dhildenb
