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
