linux-kernel - Re: [PATCH v3 0/6] add mTHP support for anonymous shmem

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Date: Sun, 2 Jun 2024 12:15:40 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, wangkefeng.wang@...wei.com, ying.huang@...el.com,
 21cnbao@...il.com, ryan.roberts@....com, shy828301@...il.com,
 ziy@...dia.com, ioworker0@...il.com, da.gomez@...sung.com,
 p.raghav@...sung.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/6] add mTHP support for anonymous shmem



On 2024/5/31 19:13, David Hildenbrand wrote:
>>>
>>> As a default, we should not be using large folios / mTHP for any shmem,
>>> just like we did with THP via shmem_enabled. This is what this series
>>> currently does, and is aprt of the whole mTHP user-space interface 
>>> design.
>>>
>>> Further, the mTHP controls should control all of shmem, not only
>>> "anonymous shmem".
>>
>> Yes, that's what I thought and in my TODO list.
> 
> Good, it would be helpful to coordinate with Daniel and Pankaj.
> 
>>
>>>
>>> Also, we should properly fallback within the configured sizes, and not
>>> jump "over" configured sizes. Unless there is a good reason.
>>>
>>> (3) khugepaged
>>>
>>> khugepaged needs to handle larger folios properly as well. Until fixed,
>>> using smaller THP sizes as fallback might prohibit collapsing a
>>> PMD-sized THP later. But really, khugepaged needs to be fixed to handle
>>> that. >
>>> (4) force/disable
>>>
>>> These settings are rather testing artifacts from the old ages. We should
>>> not add them to the per-size toggles. We might "inherit" it from the
>>> global one, though.
>>
>> Sorry, I missed this. So I thould remove the 'force' and 'deny' option
>> for each mTHP, right?
> 
> Yes, that's my understanding. But we have to keep them on the top level 
> for any possible user out there.

OK.

>>>
>>> "within_size" might have value, and especially for consistency, we
>>> should have them per size.
>>>
>>>
>>>
>>> So, this series only tackles anonymous shmem, which is a good starting
>>> point. Ideally, we'd get support for other shmem (especially during
>>> fault time) soon afterwards, because we won't be adding separate toggles
>>> for that from the interface POV, and having inconsistent behavior
>>> between kernel versions would be a bit unfortunate.
>>>
>>>
>>> @Baolin, this series likely does not consider (4) yet. And I suggest we
>>> have to take a lot of the "anonymous thp" terminology out of this
>>> series, especially when it comes to documentation.
>>
>> Sure. I will remove the "anonymous thp" terminology from the
>> documentation, but want to still keep it in the commit message, cause I
>> want to start from the anonymous shmem.
> 
> For commit message and friends makes sense. The story should be 
> "controls all of shmem/tmpfs, but support will be added iteratively. The 
> first step is anonymous shmem."

Sure. Thanks.