Re: nfsd: add the ability to enable use of RWF_DONTCACHE for all nfsd IO

[Chuck Lever <chuck.lever@oracle.com>]

On 2/21/25 10:36 AM, Mike Snitzer wrote:
> On Fri, Feb 21, 2025 at 10:25:03AM -0500, Jeff Layton wrote:
>> On Fri, 2025-02-21 at 10:02 -0500, Mike Snitzer wrote:
>>> My intent was to make 6.14's DONTCACHE feature able to be tested in
>>> the context of nfsd in a no-frills way.  I realize adding the
>>> nfsd_dontcache knob skews toward too raw, lacks polish.  But I'm
>>> inclined to expose such course-grained opt-in knobs to encourage
>>> others' discovery (and answers to some of the questions you pose
>>> below).  I also hope to enlist all NFSD reviewers' help in
>>> categorizing/documenting where DONTCACHE helps/hurts. ;)
>>>
>>> And I agree that ultimately per-export control is needed.  I'll take
>>> the time to implement that, hopeful to have something more suitable in
>>> time for LSF.
>>
>> Would it make more sense to hook DONTCACHE up to the IO_ADVISE
>> operation in RFC7862? IO_ADVISE4_NOREUSE sounds like it has similar
>> meaning? That would give the clients a way to do this on a per-open
>> basis.
> 
> Just thinking aloud here but: Using a DONTCACHE scalpel on a per open
> basis quite likely wouldn't provide the required page reclaim relief
> if the server is being hammered with normal buffered IO.  Sure that
> particular DONTCACHE IO wouldn't contribute to the problem but it
> would still be impacted by those not opting to use DONTCACHE on entry
> to the server due to needing pages for its DONTCACHE buffered IO.

For this initial work, which is to provide a mechanism for
experimentation, IMO exposing the setting to clients won't be all
that helpful.

But there are some applications/workloads on clients where exposure
could be beneficial -- for instance, a backup job, where NFSD would
benefit by knowing it doesn't have to maintain the job's written data in
its page cache. I regard that as a later evolutionary improvement,
though.

Jorge proposed adding the NFSv4.2 IO_ADVISE operation to NFSD, but I
think we first need to a) work out and document appropriate semantics
for each hint, because the spec does not provide specifics, and b)
perform some extensive benchmarking to understand their value and
impact.


-- 
Chuck Lever