Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

[Hugh Dickins <hughd@google.com>]

On Wed, 25 Jun 2025, Baolin Wang wrote:

> When invoking thp_vma_allowable_orders(), if the TVA_ENFORCE_SYSFS flag is not
> specified, we will ignore the THP sysfs settings. Whilst it makes sense for the
> callers who do not specify this flag, it creates a odd and surprising situation
> where a sysadmin specifying 'never' for all THP sizes still observing THP pages
> being allocated and used on the system. And the MADV_COLLAPSE is an example of
> such a case, that means it will not set TVA_ENFORCE_SYSFS when calling
> thp_vma_allowable_orders().
> 
> 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.
> 
> For example, system administrators who disabled THP everywhere must indeed very
> much not want THP to be used for whatever reason - having individual programs
> being able to quietly override this is very surprising and likely to cause headaches
> for those who desire this not to happen on their systems.
> 
> This patch set will address the MADV_COLLAPSE issue.
> 
> Test
> ====
> 1. Tested the mm selftests and found no regressions.
> 2. With toggling different Anon mTHP settings, the allocation and madvise collapse for
> anonymous pages work well.
> 3. With toggling different shmem mTHP settings, the allocation and madvise collapse for
> shmem work well.
> 4. Tested the large order allocation for tmpfs, and works as expected.
> 
> [1] https://lore.kernel.org/all/1f00fdc3-a3a3-464b-8565-4c1b23d34f8d@linux.alibaba.com/
> 
> Changes from v3:
>  - Collect reviewed tags. Thanks.
>  - Update the commit message, per David.
> 
> Changes from v2:
>  - Update the commit message and cover letter, per Lorenzo. Thanks.
>  - Simplify the logic in thp_vma_allowable_orders(), per Lorenzo and David. Thanks.
> 
> Changes from v1:
>  - Update the commit message, per Zi.
>  - Add Zi's reviewed tag. Thanks.
>  - Update the shmem logic.
> 
> Baolin Wang (2):
>   mm: huge_memory: disallow hugepages if the system-wide THP sysfs
>     settings are disabled
>   mm: shmem: disallow hugepages if the system-wide shmem THP sysfs
>     settings are disabled
> 
>  include/linux/huge_mm.h                 | 51 ++++++++++++++++++-------
>  mm/shmem.c                              |  6 +--
>  tools/testing/selftests/mm/khugepaged.c |  8 +---
>  3 files changed, 43 insertions(+), 22 deletions(-)
> 
> -- 
> 2.43.5

Sorry for chiming in so late, after so much effort: but I beg you,
please drop these.

I did not want to get into a fight, and had been hoping a voice of
reason would come from others, before I got around to responding.

And indeed Ryan understood correctly at the start; and he, Usama
and Barry, perhaps others I've missed, have raised appropriate
concerns but not prevailed.

If we're sloganeering, I much prefer "never break userspace" to
"never means never", attractive though that over-simplification is.

Seldom has a feature been so thorougly documented as MADV_COLLAPSE,
in its 6.1 commits and in the "man 2 madvise" page: which are
explicit about MADV_COLLAPSE providing a way to get THPs where the
sysfs setting governing automatic behaviour does not insert them.

We would all prefer a less messy world of THP tunables.  I certainly
find plenty to dislike there too; and wish that a less assertive name
than "never" had been chosen originally for the default off position.

But please don't break the accepted and documented behaviour of
MADV_COLLAPSE now.

If you want to exclude all possibility of THPs, then please use the
prctl(PR_SET_THP_DISABLE); or shmem_enabled=deny (I think it was me
who insisted that be respected by MADV_COLLAPSE back then).

Add a "deny" option to /sys/kernel/mm/transparent_hugepage/enabled
if you like.  (But in these days of filesystem large folios, adding
new protections against them seems a few years late.)

If Andrew decides that these patches should go in, then I'll have to
scrutinize them more carefully than I've done so far: but currently
I'm hoping to avoid that.

Hugh