linux-kernel - Re: [PATCH v2 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

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Message-ID: <cb6d3f39-e0a2-4618-b36d-fff8724bf619@gmail.com>
Date: Fri, 13 Jun 2025 15:23:19 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com, david@...hat.com
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] fix MADV_COLLAPSE issue if THP settings are
 disabled

On 05/06/2025 09:00, 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.

Hi Baolin,

I know never means never, but I also thought that the per-size toggles had
priority over the system ones. This was discussed in [1] as well.

My understanding with these patches is that if we have:

[root@vm4 vmuser]# cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]
[root@vm4 vmuser]# cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
always inherit [madvise] never

Than without these patches we get a hugepage when we do MADV_HUGEPAGE, but with
these we won't get a hugepage anymore eventhough hugepages-2048kB/enabled is set
to madvise?

I know this isn't ABI, but this would break existing expectations.
(For e.g. we have certain 64K page size arm machines with global enabled = never and
2M = madvise, and we want 2M hugepages to fault at madvise).
If the whole thing was being implemented from scratch, we should have definitely
done it this way, but this can give a people a nasty surprise when they upgrade
the kernel and suddenly stop getting hugepages.

[1] https://lore.kernel.org/all/97702ff0-fc50-4779-bfa8-83dc42352db1@redhat.com/

Thanks,
Usama