Message-ID: <8981f5a6-670d-4d01-8eca-d18fd285add8@arm.com>
Date: Wed, 25 Jun 2025 11:35:08 +0530
From: Dev Jain <dev.jain@....com>
To: Hugh Dickins <hughd@...gle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, baohua@...nel.org, zokeefe@...gle.com,
 shy828301@...il.com, usamaarif642@...il.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are
 disabled


On 25/06/25 11:23 am, 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.
>
> 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.

FWIW I had the same thought of "never break userspace" when I first
saw the patch but then this point did not shine out in the discussion
so I became nervous to mention it again :)

>
> 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

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