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

[Dev Jain <dev.jain@arm.com>]

On 25/06/25 11:56 am, Baolin Wang wrote:
>
>
> On 2025/6/25 13:53, Hugh Dickins wrote:
>> 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.
>
> Thanks Hugh for your input. (yes, we put in a lot of effort on 
> discussion and testing:( ).
>
>> 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.
>
> Yes, agree. we should not break userspace, however, I suspect whether 
> this can really break userspace. We can set 
> '/sys/kernel/mm/transparent_hugepage/enabled' to 'madvise' to allow 
> MADV_COLLAPSE. Additionally, I really doubt that when the system-wide 
> THP settings are set to 'never', userspace would still expect to 
> collapse into THP using MADV_COLLAPSE.

After this patch, will a user still be able to use MADV_COLLAPSE and 
ensure no interference from khugepaged?


>
> Moreover, what makes this issue particularly frustrating is that when 
> we introduce mTHP collapse[1], MADV_COLLAPSE complicates matters 
> further. That is, when the system only enables 64K mTHP, MADV_COLLAPSE 
> still allows collapsing into PMD-sized THP. This really breaks the 
> user's settings.

This issue will still be there without this patch right?


>
> [1] 
> https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
>
>> 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).
>
> Yes, that will prevent MADV_COLLAPSE.
>
>> 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
>