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

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Message-ID: <16ef3751-1837-4485-bf9b-33b0e709528f@arm.com>
Date: Fri, 5 Jul 2024 10:23:37 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.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 05/07/2024 10:16, David Hildenbrand wrote:
> 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"

I definitely prefer the direct translation. :)