Re: [PATCH v5 4/6] mm: shmem: add mTHP support for anonymous shmem

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/7/4 22:46, Bang Li wrote:
> Hi Bao lin,
> 
> On 2024/7/4 19:15, Baolin Wang wrote:
>>
>>>> +
>>>> +    /*
>>>> +     * Only allow inherit orders if the top-level value is 'force', 
>>>> which
>>>> +     * means non-PMD sized THP can not override 'huge' mount option 
>>>> now.
>>>> +     */
>>>> +    if (shmem_huge == SHMEM_HUGE_FORCE)
>>>> +        return READ_ONCE(huge_shmem_orders_inherit);
>>>
>>> I vaguely recall that we originally discussed that trying to set 
>>> 'force' on the
>>> top level control while any per-size controls were set to 'inherit' 
>>> would be an
>>> error, and trying to set 'force' on any per-size control except the 
>>> PMD-size
>>> would be an error?
>>
>> Right.
>>
>>> I don't really understand this logic. Shouldn't we just be looking at 
>>> the
>>> per-size control settings (or the top-level control as a proxy for every
>>> per-size control that has 'inherit' set)?
>>
>> ‘force’ will apply the huge orders for anon shmem and tmpfs, so now we 
>> only allow pmd-mapped THP to be forced. We should not look at per-size 
>> control settings for tmpfs now (mTHP for tmpfs will be discussed in 
>> future).
>>
>>>
>>> Then for tmpfs, which doesn't support non-PMD-sizes yet, we just 
>>> always use the
>>> PMD-size control for decisions.
>>>
>>> I'm also really struggling with the concept of shmem_is_huge() 
>>> existing along
>>> side shmem_allowable_huge_orders(). Surely this needs to all be 
>>> refactored into
>>> shmem_allowable_huge_orders()?
>>
>> I understood. But now they serve different purposes: shmem_is_huge() 
>> will be used to check the huge orders for the top level, for *tmpfs* 
>> and anon shmem; whereas shmem_allowable_huge_orders() will only be 
>> used to check the per-size huge orders for anon shmem (excluding tmpfs 
>> now). However, as I plan to add mTHP support for tmpfs, I think we can 
>> perform some cleanups. 
> 
> Please count me in, I'd be happy to contribute to the cleanup and 
> enhancement
> process if I can.

Good. If you have time, I think you can look at the shmem khugepaged 
issue from the previous discussion [1], which I don't have time to look 
at now.

"
(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.
"

[1] 
https://lore.kernel.org/all/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com/T/#u