Re: [RFC PATCH v3 0/4] Support large folios for tmpfs

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

On 2024/10/16 21:45, Kefeng Wang wrote:
> 
> 
> On 2024/10/16 17:29, Baolin Wang wrote:
>>
>>
>> On 2024/10/16 15:49, Kefeng Wang wrote:
>>>
>>>
>>> On 2024/10/10 17:58, Baolin Wang wrote:
>>>> Hi,
>>>>
>>>> This RFC patch series attempts to support large folios for tmpfs.
>>>>
>>>> Considering that tmpfs already has the 'huge=' option to control the 
>>>> THP
>>>> allocation, it is necessary to maintain compatibility with the 'huge='
>>>> option, as well as considering the 'deny' and 'force' option controlled
>>>> by '/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
>>>>
>>>> Add a new huge option 'write_size' to support large folio allocation 
>>>> based
>>>> on the write size for tmpfs write and fallocate paths. So the huge 
>>>> pages
>>>> allocation strategy for tmpfs is that, if the 'huge=' option
>>>> (huge=always/within_size/advise) is enabled or the 'shmem_enabled' 
>>>> option
>>>> is 'force', it need just allow PMD sized THP to keep backward 
>>>> compatibility
>>>> for tmpfs. While 'huge=' option is disabled (huge=never) or the 
>>>> 'shmem_enabled'
>>>> option is 'deny', it will still disable any large folio allocations. 
>>>> Only
>>>> when the 'huge=' option is 'write_size', it will allow allocating large
>>>> folios based on the write size.
>>>>
>>>> And I think the 'huge=write_size' option should be the default behavior
>>>> for tmpfs in future.
>>>
>>> Could we avoid new huge= option for tmpfs, maybe support other orders
>>> for both read/write/fallocate if mount with huge?
>>
>> Um, I am afraid not, as that would break the 'huge=' compatibility. 
>> That is to say, users still want PMD-sized huge pages if 'huge=always'.
> 
> Yes, compatibility maybe an issue, but only write/fallocate side support
> large folio is a little strange, maybe a new mode to support both read/
> write/fallocate?

Because tmpfs read() will not allocate folios for tmpfs holes, and will 
use ZERO_PAGE instead. If the shmem folios are swapped out, and now we 
will always swapin base page, which is another story...

For tmpfs mmap() read, we do not have a length to indicate how large the 
folio should be allocated. Moreover, we have decided against adding any 
mTHP interfaces for tmpfs in the previous discussion[1].

[1] https://lore.kernel.org/all/ZvVRiJYfaXD645Nh@casper.infradead.org/