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

[Ryan Roberts <ryan.roberts@arm.com>]

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?

> 
> But now we already have a PMD-mapped THP control for tmpfs, and mTHP simply
> extends this control to per-size.
> 
> IIUC, as David mentioned before, for tmpfs, mTHP should act like a huge order
> filter which should be respected by the expected huge orders in the write() and
> fallocate() paths. This would also solve the issue of allocating huge orders in
> writable mmap() path for tmpfs, as well as unifying the interface.
> 
> Anyway, I will try to provide an RFC to discuss the mTHP for tmpfs approach.