Re: [PATCH v7 06/12] khugepaged: introduce khugepaged_scan_bitmap for mTHP support

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

On 2025/5/28 17:26, David Hildenbrand wrote:
> On 22.05.25 11:39, Baolin Wang wrote:
>>
>>
>> On 2025/5/21 18:23, Nico Pache wrote:
>>> On Tue, May 20, 2025 at 4:09 AM Baolin Wang
>>> <baolin.wang@linux.alibaba.com> wrote:
>>>>
>>>> Sorry for late reply.
>>>>
>>>> On 2025/5/17 14:47, Nico Pache wrote:
>>>>> On Thu, May 15, 2025 at 9:20 PM Baolin Wang
>>>>> <baolin.wang@linux.alibaba.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 2025/5/15 11:22, Nico Pache wrote:
>>>>>>> khugepaged scans anons PMD ranges for potential collapse to a 
>>>>>>> hugepage.
>>>>>>> To add mTHP support we use this scan to instead record chunks of 
>>>>>>> utilized
>>>>>>> sections of the PMD.
>>>>>>>
>>>>>>> khugepaged_scan_bitmap uses a stack struct to recursively scan a 
>>>>>>> bitmap
>>>>>>> that represents chunks of utilized regions. We can then determine 
>>>>>>> what
>>>>>>> mTHP size fits best and in the following patch, we set this 
>>>>>>> bitmap while
>>>>>>> scanning the anon PMD. A minimum collapse order of 2 is used as 
>>>>>>> this is
>>>>>>> the lowest order supported by anon memory.
>>>>>>>
>>>>>>> max_ptes_none is used as a scale to determine how "full" an order 
>>>>>>> must
>>>>>>> be before being considered for collapse.
>>>>>>>
>>>>>>> When attempting to collapse an order that has its order set to 
>>>>>>> "always"
>>>>>>> lets always collapse to that order in a greedy manner without
>>>>>>> considering the number of bits set.
>>>>>>>
>>>>>>> Signed-off-by: Nico Pache <npache@redhat.com>
>>>>>>
>>>>>> Sigh. You still haven't addressed or explained the issues I 
>>>>>> previously
>>>>>> raised [1], so I don't know how to review this patch again...
>>>>> Can you still reproduce this issue?
>>>>
>>>> Yes, I can still reproduce this issue with today's (5/20) mm-new 
>>>> branch.
>>>>
>>>> I've disabled PMD-sized THP in my system:
>>>> [root]# cat /sys/kernel/mm/transparent_hugepage/enabled
>>>> always madvise [never]
>>>> [root]# cat 
>>>> /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
>>>> always inherit madvise [never]
>>>>
>>>> And I tried calling madvise() with MADV_COLLAPSE for anonymous memory,
>>>> and I can still see it collapsing to a PMD-sized THP.
>>> Hi Baolin ! Thank you for your reply and willingness to test again :)
>>>
>>> I didn't realize we were talking about madvise collapse-- this makes
>>> sense now. I also figured out why I could "reproduce" it before. My
>>> script was always enabling the THP settings in two places, and I only
>>> commented out one to test this. But this time I was doing more manual
>>> testing.
>>>
>>> The original design of madvise_collapse ignores the sysfs and
>>> collapses even if you have an order disabled. I believe this behavior
>>> is wrong, but by design. I spent some time playing around with madvise
>>> collapses with and w/o my changes. This is not a new thing, I
>>> reproduced the issue in 6.11 (Fedora 41), and I think its been
>>> possible since the inception of madvise collapse 3 years ago. I
>>> noticed a similar behavior on one of my RFC since it was "breaking"
>>> selftests, and the fix was to reincorporate this broken sysfs
>>> behavior.
>>
>> OK. Thanks for the explanation.
>>
>>> 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage 
>>> collapse")
>>> "This call is independent of the system-wide THP sysfs settings, but
>>> will fail for memory marked VM_NOHUGEPAGE."
>>>
>>> The second condition holds true (and fails for VM_NOHUGEPAGE), but I
>>> dont know if we actually want madvise_collapse to be independent of
>>> the system-wide.
>>
>> This design principle surprised me a bit, and I failed to find the
>> reason in the commit log. I agree that "never should mean never," and we
>> should respect the THP/mTHP sysfs setting. Additionally, for the
>> 'shmem_enabled' sysfs interface controlled for shmem/tmpfs, THP collapse
>> can still be prohibited through the 'deny' configuration. The rules here
>> are somewhat confusing.
> 
> I recall that we decided to overwrite "VM_NOHUGEPAGE", because the 
> assumption is that the same app that triggered MADV_NOHUGEPAGE triggers 
> the collapse. So the app decides on its own behavior.
> 
> Similarly, allowing for collapsing in a VM without VM_HUGEPAGE in the 
> "madvise" mode would be fine.
> 
> But in the "never" case, we should just "never" collapse.

OK. Let's fix the "never" case first. Thanks.