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

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

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.

>
> Kirill, Hugh othe folks?