Re: [PATCH v8 3/7] NFSD: add io_cache_read controls to debugfs interface

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Sep 03, 2025 at 11:07:29AM -0400, Mike Snitzer wrote:
> On Wed, Sep 03, 2025 at 10:38:45AM -0400, Chuck Lever wrote:

> > > diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
> > > index 1cd0bed57bc2f..6ef799405145f 100644
> > > --- a/fs/nfsd/nfsd.h
> > > +++ b/fs/nfsd/nfsd.h
> > > @@ -153,6 +153,15 @@ static inline void nfsd_debugfs_exit(void) {}
> > >  
> > >  extern bool nfsd_disable_splice_read __read_mostly;
> > >  
> > > +enum {
> > > +	NFSD_IO_UNSPECIFIED = 0,
> > > +	NFSD_IO_BUFFERED,
> > > +	NFSD_IO_DONTCACHE,
> > > +	NFSD_IO_DIRECT,
> > > +};
> > > +
> > > +extern u64 nfsd_io_cache_read __read_mostly;
> > 
> > And then here, initialize nfsd_io_cache_read to reflect the default
> > behavior. That would be NFSD_IO_BUFFERED for now... then later we might
> > want to change it to NFSD_IO_DIRECT, for instance.
> > 
> > Same suggestion for 4/7.
> 
> Ah ok, I can see the way forward to default to NFSD_IO_BUFFERED but
> _not_ default to it when erroring (if the user specified some unknown
> value).
> 
> I'll run with that (despite just asking Jeff's opinion above, I'm the
> one who came up with the awkward UNSPECIFIED state when honoring
> Jeff's early feedback).

Here is the incremental diff (these changes will be folded into
appropriate patches in v9):

diff --git a/fs/nfsd/debugfs.c b/fs/nfsd/debugfs.c
index 8878c3519b30c..173032a04cdec 100644
--- a/fs/nfsd/debugfs.c
+++ b/fs/nfsd/debugfs.c
@@ -43,11 +43,10 @@ DEFINE_DEBUGFS_ATTRIBUTE(nfsd_dsr_fops, nfsd_dsr_get, nfsd_dsr_set, "%llu\n");
  * /sys/kernel/debug/nfsd/io_cache_read
  *
  * Contents:
- *   %1: NFS READ will use buffered IO
- *   %2: NFS READ will use dontcache (buffered IO w/ dropbehind)
- *   %3: NFS READ will use direct IO
+ *   %0: NFS READ will use buffered IO
+ *   %1: NFS READ will use dontcache (buffered IO w/ dropbehind)
+ *   %2: NFS READ will use direct IO
  *
- * The default value of this setting is zero (UNSPECIFIED).
  * This setting takes immediate effect for all NFS versions,
  * all exports, and in all NFSD net namespaces.
  */
@@ -90,11 +89,10 @@ DEFINE_DEBUGFS_ATTRIBUTE(nfsd_io_cache_read_fops, nfsd_io_cache_read_get,
  * /sys/kernel/debug/nfsd/io_cache_write
  *
  * Contents:
- *   %1: NFS WRITE will use buffered IO
- *   %2: NFS WRITE will use dontcache (buffered IO w/ dropbehind)
- *   %3: NFS WRITE will use direct IO
+ *   %0: NFS WRITE will use buffered IO
+ *   %1: NFS WRITE will use dontcache (buffered IO w/ dropbehind)
+ *   %2: NFS WRITE will use direct IO
  *
- * The default value of this setting is zero (UNSPECIFIED).
  * This setting takes immediate effect for all NFS versions,
  * all exports, and in all NFSD net namespaces.
  */
diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
index fe935b4cda538..412a1e9a2a876 100644
--- a/fs/nfsd/nfsd.h
+++ b/fs/nfsd/nfsd.h
@@ -154,8 +154,7 @@ static inline void nfsd_debugfs_exit(void) {}
 extern bool nfsd_disable_splice_read __read_mostly;
 
 enum {
-	NFSD_IO_UNSPECIFIED = 0,
-	NFSD_IO_BUFFERED,
+	NFSD_IO_BUFFERED = 0,
 	NFSD_IO_DONTCACHE,
 	NFSD_IO_DIRECT,
 };
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 5e700a0d6b12e..403076443573f 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -50,8 +50,8 @@
 #define NFSDDBG_FACILITY		NFSDDBG_FILEOP
 
 bool nfsd_disable_splice_read __read_mostly;
-u64 nfsd_io_cache_read __read_mostly;
-u64 nfsd_io_cache_write __read_mostly;
+u64 nfsd_io_cache_read __read_mostly = NFSD_IO_BUFFERED;
+u64 nfsd_io_cache_write __read_mostly = NFSD_IO_BUFFERED;
 
 /**
  * nfserrno - Map Linux errnos to NFS errnos
@@ -1272,8 +1272,7 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
 		break;
 	case NFSD_IO_DONTCACHE:
 		kiocb.ki_flags = IOCB_DONTCACHE;
-		fallthrough;
-	case NFSD_IO_UNSPECIFIED:
+		break;
 	case NFSD_IO_BUFFERED:
 		break;
 	}
@@ -1605,8 +1604,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
 		break;
 	case NFSD_IO_DONTCACHE:
 		kiocb.ki_flags |= IOCB_DONTCACHE;
-		fallthrough;
-	case NFSD_IO_UNSPECIFIED:
+		fallthrough; /* must call nfsd_issue_write_buffered */
 	case NFSD_IO_BUFFERED:
 		host_err = nfsd_issue_write_buffered(rqstp, file,
 						nvecs, cnt, &kiocb);