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

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Message-ID: <00999fc3-3a4a-4ee5-8021-81c73253fe7f@arm.com>
Date: Fri, 30 May 2025 10:07:11 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.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 09:52, David Hildenbrand wrote:
> On 30.05.25 10:47, Lorenzo Stoakes wrote:
>> On Fri, May 30, 2025 at 10:44:36AM +0200, 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?
>>>
>>> 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).
>>>
>>
>> I'm with David on this one.
>>
>> I think it's principal of least surprise - to me 'never' is pretty
>> emphatic, and keep in mind the other choices are 'always' and...  'madvise'
>> :) which explicitly is 'hey only do this if madvise tells you to'.

I think it's a bit reductive to suggest that enabled=madvise means all madvise
calls though. I don't think anyone would suggest MADV_DONTNEED should be ignored
if enabled=never. MADV_COLLAPSE just happens to be implemented on top of the THP
logic. But it's a different feature in my view.

>>
>> I'd be rather surprised if I hadn't set madvise and a user uses madvise to
>> in some fashion override the never.
>>
>> I mean I think we all agree this interface is to use a technical term -
>> crap - and we need something a lot more fine-grained and smart, 

Yes agreed there!

>> but I think
>> given the situation we're in we should make it at least as least surprising
>> as possible.

> 
> Yes. If you configure "never" you are supposed to suffer, consistently.
> 

OK fair enough. Just giving my 2 cents.