linux-kernel - Re: [PATCH v5 0/6] add mTHP support for anonymous shmem

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Message-ID: <ced474d1-86b7-471a-aee9-6e4722eb1471@redhat.com>
Date: Fri, 5 Jul 2024 11:16:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, wangkefeng.wang@...wei.com,
 ying.huang@...el.com, 21cnbao@...il.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 v5 0/6] add mTHP support for anonymous shmem

On 05.07.24 11:13, Ryan Roberts wrote:
> On 05/07/2024 09:59, David Hildenbrand wrote:
>> On 05.07.24 10:45, Ryan Roberts wrote:
>>> On 05/07/2024 06:47, Baolin Wang wrote:
>>>>
>>>>
>>>> On 2024/7/5 03:49, Matthew Wilcox wrote:
>>>>> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote:
>>>>>> On 04.07.24 21:03, David Hildenbrand wrote:
>>>>>>>> shmem has two uses:
>>>>>>>>
>>>>>>>>       - MAP_ANONYMOUS | MAP_SHARED (this patch set)
>>>>>>>>       - tmpfs
>>>>>>>>
>>>>>>>> For the second use case we don't want controls *at all*, we want the
>>>>>>>> same heiristics used for all other filesystems to apply to tmpfs.
>>>>>>>
>>>>>>> As discussed in the MM meeting, Hugh had a different opinion on that.
>>>>>>
>>>>>> FWIW, I just recalled that I wrote a quick summary:
>>>>>>
>>>>>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com
>>>>>>
>>>>>> I believe the meetings are recorded as well, but never looked at recordings.
>>>>>
>>>>> That's not what I understood Hugh to mean.  To me, it seemed that Hugh
>>>>> was expressing an opinion on using shmem as shmem, not as using it as
>>>>> tmpfs.
>>>>>
>>>>> If I misunderstood Hugh, well, I still disagree.  We should not have
>>>>> separate controls for this.  tmpfs is just not that special.
>>>
>>> I wasn't at the meeting that's being referred to, but I thought we previously
>>> agreed that tmpfs *is* special because in some configurations its not backed by
>>> swap so is locked in ram?
>>
>> There are multiple things to that, like:
>>
>> * Machines only having limited/no swap configured
>> * tmpfs can be configured to never go to swap
>> * memfd/tmpfs files getting used purely for mmap(): there is no real
>>    difference to MAP_ANON|MAP_SHARE besides the processes we share that
>>    memory with.
>>
>> Especially when it comes to memory waste concerns and access behavior in some
>> cases, tmpfs behaved much more like anonymous memory. But there are for sure
>> other use cases where tmpfs is not that special.
>>
>> My opinion is that we need to let people configure orders (if you feel like it,
>> configure all), but *select* the order to allocate based on readahead
>> information -- in contrast to anonymous memory where we start at the highest
>> order and don't have readahead information available.
> 
> That approach is exactly what I proposed to start playing with yesterday [1] for
> regular pagecache folio allocations too :)

In German, there is this saying "zwei Dumme ein Gedanke".

The official English alternative is "great minds think alike".

... well, the direct German->English translation definitely has a 
"German touch" to it: "two stupid ones one thought"

-- 
Cheers,

David / dhildenb