Re: [PATCH 1/6] NFSD: add the ability to enable use of RWF_DONTCACHE for all IO

Chuck Lever
2025-06-16T13:32:26Z

On 6/12/25 12:00 PM, Mike Snitzer wrote:
> On Thu, Jun 12, 2025 at 09:21:35AM -0400, Chuck Lever wrote:
>> On 6/11/25 3:18 PM, Mike Snitzer wrote:
>>> On Wed, Jun 11, 2025 at 10:31:20AM -0400, Chuck Lever wrote:
>>>> On 6/10/25 4:57 PM, Mike Snitzer wrote:
>>>>> Add 'enable-dontcache' to NFSD's debugfs interface so that: Any data
>>>>> read or written by NFSD will either not be cached (thanks to O_DIRECT)
>>>>> or will be removed from the page cache upon completion (DONTCACHE).
>>>>
>>>> I thought we were going to do two switches: One for reads and one for
>>>> writes? I could be misremembering.
>>>
>>> We did discuss the possibility of doing that. Still can-do if that's
>>> what you'd prefer.
>>
>> For our experimental interface, I think having read and write enablement
>> as separate settings is wise, so please do that.
>>
>> One quibble, though: The name "enable_dontcache" might be directly
>> meaningful to you, but I think others might find "enable_dont" to be
>> oxymoronic. And, it ties the setting to a specific kernel technology:
>> RWF_DONTCACHE.
>>
>> So: Can we call these settings "io_cache_read" and "io_cache_write" ?
>>
>> They could each carry multiple settings:
>>
>> 0: Use page cache
>> 1: Use RWF_DONTCACHE
>> 2: Use O_DIRECT
>>
>> You can choose to implement any or all of the above three mechanisms.
>
> I like it, will do for v2. But will have O_DIRECT=1 and RWF_DONTCACHE=2.

For io_cache_read, either settings 1 and 2 need to set
disable_splice_read, or the io_cache_read setting has to be considered
by nfsd_read_splice_ok() when deciding to use nfsd_iter_read() or
splice read.

However, it would be slightly nicer if we could decide whether splice
read can be removed /before/ this series is merged. Can you get NFSD
tested with IOR with disable_splice_read both enabled and disabled (no
direct I/O)? Then we can compare the results to ensure that there is no
negative performance impact for removing the splice read code.

--
Chuck Lever