Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility

[Yang Shi <yang.shi@linux.alibaba.com>]

On 6/7/19 3:57 AM, Hugh Dickins wrote:
> On Thu, 6 Jun 2019, Yang Shi wrote:
>> On 5/7/19 10:10 AM, Yang Shi wrote:
>>> On 5/7/19 3:47 AM, Michal Hocko wrote:
>>>> [Hmm, I thought, Hugh was CCed]
>>>>
>>>> On Mon 06-05-19 16:37:42, Yang Shi wrote:
>>>>> On 4/28/19 12:13 PM, Yang Shi wrote:
>>>>>> On 4/23/19 10:52 AM, Michal Hocko wrote:
>>>>>>> On Wed 24-04-19 00:43:01, Yang Shi wrote:
>>>>>>>> The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility
>>>>>>>> for each
>>>>>>>> vma") introduced THPeligible bit for processes' smaps. But, when
>>>>>>>> checking
>>>>>>>> the eligibility for shmem vma, __transparent_hugepage_enabled()
>>>>>>>> is
>>>>>>>> called to override the result from shmem_huge_enabled().  It may
>>>>>>>> result
>>>>>>>> in the anonymous vma's THP flag override shmem's.  For example,
>>>>>>>> running a
>>>>>>>> simple test which create THP for shmem, but with anonymous THP
>>>>>>>> disabled,
>>>>>>>> when reading the process's smaps, it may show:
>>>>>>>>
>>>>>>>> 7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test
>>>>>>>> Size:               4096 kB
>>>>>>>> ...
>>>>>>>> [snip]
>>>>>>>> ...
>>>>>>>> ShmemPmdMapped:     4096 kB
>>>>>>>> ...
>>>>>>>> [snip]
>>>>>>>> ...
>>>>>>>> THPeligible:    0
>>>>>>>>
>>>>>>>> And, /proc/meminfo does show THP allocated and PMD mapped too:
>>>>>>>>
>>>>>>>> ShmemHugePages:     4096 kB
>>>>>>>> ShmemPmdMapped:     4096 kB
>>>>>>>>
>>>>>>>> This doesn't make too much sense.  The anonymous THP flag should
>>>>>>>> not
>>>>>>>> intervene shmem THP.  Calling shmem_huge_enabled() with checking
>>>>>>>> MMF_DISABLE_THP sounds good enough.  And, we could skip stack and
>>>>>>>> dax vma check since we already checked if the vma is shmem
>>>>>>>> already.
>>>>>>> Kirill, can we get a confirmation that this is really intended
>>>>>>> behavior
>>>>>>> rather than an omission please? Is this documented? What is a
>>>>>>> global
>>>>>>> knob to simply disable THP system wise?
>>>>>> Hi Kirill,
>>>>>>
>>>>>> Ping. Any comment?
>>>>> Talked with Kirill at LSFMM, it sounds this is kind of intended
>>>>> behavior
>>>>> according to him. But, we all agree it looks inconsistent.
>>>>>
>>>>> So, we may have two options:
>>>>>       - Just fix the false negative issue as what the patch does
>>>>>       - Change the behavior to make it more consistent
>>>>>
>>>>> I'm not sure whether anyone relies on the behavior explicitly or
>>>>> implicitly
>>>>> or not.
>>>> Well, I would be certainly more happy with a more consistent behavior.
>>>> Talked to Hugh at LSFMM about this and he finds treating shmem objects
>>>> separately from the anonymous memory. And that is already the case
>>>> partially when each mount point might have its own setup. So the primary
>>>> question is whether we need a one global knob to controll all THP
>>>> allocations. One argument to have that is that it might be helpful to
>>>> for an admin to simply disable source of THP at a single place rather
>>>> than crawling over all shmem mount points and remount them. Especially
>>>> in environments where shmem points are mounted in a container by a
>>>> non-root. Why would somebody wanted something like that? One example
>>>> would be to temporarily workaround high order allocations issues which
>>>> we have seen non trivial amount of in the past and we are likely not at
>>>> the end of the tunel.
>>> Shmem has a global control for such use. Setting shmem_enabled to "force"
>>> or "deny" would enable or disable THP for shmem globally, including non-fs
>>> objects, i.e. memfd, SYS V shmem, etc.
>>>
>>>> That being said I would be in favor of treating the global sysfs knob to
>>>> be global for all THP allocations. I will not push back on that if there
>>>> is a general consensus that shmem and fs in general are a different
>>>> class of objects and a single global control is not desirable for
>>>> whatever reasons.
>>> OK, we need more inputs from Kirill, Hugh and other folks.
>> [Forgot cc to mailing lists]
>>
>> Hi guys,
>>
>> How should we move forward for this one? Make the sysfs knob
>> (/sys/kernel/mm/transparent_hugepage/enabled) to be global for both anonymous
>> and tmpfs? Or just treat shmem objects separately from anon memory then fix
>> the false-negative of THP eligibility by this patch?
> Sorry for not getting back to you sooner on this.
>
> I don't like to drive design by smaps. I agree with the word "mess" used
> several times of THP tunings in this thread, but it's too easy to make
> that mess worse by unnecessary changes, so I'm very cautious here.
>
> The addition of "THPeligible" without an "Anon" in its name was
> unfortunate. I suppose we're two releases too late to change that.

The smaps shows it is anon vma or shmem vma for the most cases.

>
> Applying process (PR_SET_THP_DISABLE) and mm (MADV_*HUGEPAGE)
> limitations to shared filesystem objects doesn't work all that well.

The THP eligibility indicator is per vma, it just reports whether THP is 
eligible for a specific vma. So, I'm supposed it should keep consistent 
with MMF_DISABLE_THP and MADV_*HUGEPAGE setting.

The current implementation in shmem and kuhugepaged also checks these.

>
> I recommend that you continue to treat shmem objects separately from
> anon memory, and just make the smaps "THPeligible" more often accurate.
>
> Is your v2 patch earlier in this thread the best for that?

The v2 patch treats shmem objects separately from anon memory and it 
makes the "THPeligible" more often accurate.

> No answer tonight, I'll re-examine later in the day.
>
> Hugh