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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/7/9 20:36, Lorenzo Stoakes wrote:
> +cc Pedro as he'd raised concerns here also.
> 
> Hi Baolin,
> 
> Just for some clarification on this - thank you very much for this series,
> but based on discussion with David and concerns raised by Hugh + others,
> overall it feels as if, while the documentation is no doubt vague in ways
> it ought not to be, this behaviour is something we have put out into the
> world and we should continue to support it.
> 
> So overall I feel that this series should not be applied.

Fair enough.

> 
> Your work here is great, and really massive apologies for this after all
> the work you've put in (and of course the review work here also), but on
> reflection I think it's a risk we shouldn't take.

Consensus is the key. Thank you and David for the discussion and 
suggestions.

> I understand this means that MADV_COLLAPSE can't be used to collapse at a
> mTHP granularity - we definitely need to have a think about how we might
> provide this sensibly.
> 
> As for how to move forward - I will go ahead and update documentation to
> make the situation absolutely crystal clear, both in the man page and the
> rst.

OK. Great. Thanks.

> Thanks, Lorenzo
> 
> On Wed, Jun 25, 2025 at 09:40:08AM +0800, 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
>>