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

[David Hildenbrand <david@redhat.com>]

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.

Maybe we need different "order allcoation" logic for read/write vs. 
fault, not sure.

But I don't maintain that code, so I can only give stupid suggestions 
and repeat what I understood from the meeting with Hugh and Kirill :)

-- 
Cheers,

David / dhildenb