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Subject: Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP
 eligibility
From:   Yang Shi <yang.shi@linux.alibaba.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        vbabka@suse.cz, rientjes@google.com, kirill@shutemov.name,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Hugh Dickins <hughd@google.com>
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Message-ID: <217fc290-5800-31de-7d46-aa5c0f7b1c75@linux.alibaba.com>
Date:   Thu, 6 Jun 2019 11:59:21 -0700
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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?

>
>>
>> Kirill, Hugh othe folks?
>