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

Chuck Lever
2025-06-12T13:21:44Z

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.

>> After all, you are describing two different facilities here: a form of
>> direct I/O for READs, and RWF_DONTCACHE for WRITEs (I think?).
>
> My thinking was NFSD doesn't need to provide faithful pure
> RWF_DONTCACHE if it really doesn't make sense. But the "dontcache"
> name can be (ab)used by NFSD to define it how it sees fit (O_DIRECT
> doesn't cache so it seems fair). What I arrived at with this patchset
> is how I described in my cover letter:
>
> When 'enable-dontcache' is used:
> - all READs will use O_DIRECT (both DIO-aligned and misaligned)
> - all DIO-aligned WRITEs will use O_DIRECT (useful for SUNRPC RDMA)
> - misaligned WRITEs currently continue to use normal buffered IO
>
> But we reserve the right to iterate on the implementation details as
> we see fit. Still using the umbrella of 'dontcache'.

--
Chuck Lever