linux-kernel - Re: [PATCH 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

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Message-ID: <9b1bac6c-fd9f-4dc1-8c94-c4da0cbb9e7f@arm.com>
Date: Fri, 30 May 2025 09:59:39 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

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.

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.

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?). Is there any concern that there
are workloads in such environments that are using MADV_COLLAPSE today that would
then see a performance drop?