Fix a serious deadlock with the NFS client. Given a large enough

atomic write request, it can fill the buffer cache with the entirety
of that write in order to handle retries.  However, it never drops
the vnode lock, or else it wouldn't be atomic, so it ends up waiting
indefinitely for more buf memory that cannot be gotten as it has it
all, and it waits in an uncancellable state.

To fix this, hibufspace is exported and scaled to a reasonable
fraction.  This is used as the limit of how much of an atomic write
request by the NFS client will be handled asynchronously.  If the
request is larger than this, it will be turned into a synchronous
request which won't deadlock the system.  It's possible this value is
far off from what is required by some, so it shall be tunable as soon
as mount_nfs(8) learns of the new field.

The slowdown between an asynchronous and a synchronous write on NFS
appears to be on the order of 2x-4x.

General nod by:	gad
MFC after:	2 weeks
More testing:	wes
PR:		kern/79208

1 files changed, 2 insertions, 2 deletions

diff --git a/sys/nfsclient/nfsargs.h b/sys/nfsclient/nfsargs.h
index 8aa2392..7ebf1a0 100644
--- a/sys/nfsclient/nfsargs.h
+++ b/sys/nfsclient/nfsargs.h
@@ -56,7 +56,7 @@ struct nfs_args {
 	int		retrans;	/* times to retry send */
 	int		maxgrouplist;	/* Max. size of group list */
 	int		readahead;	/* # of blocks to readahead */
-	int		__pad1;		/* was "leaseterm" */
+	int		wcommitsize;	/* Max. write commit size in bytes */
 	int		deadthresh;	/* Retrans threshold */
 	char		*hostname;	/* server's name */
 	int		acregmin;	/* cache attrs for reg files min time */
@@ -80,7 +80,7 @@ struct nfs_args {
 #define	NFSMNT_NFSV3		0x00000200  /* Use NFS Version 3 protocol */
 /* 0x400 free, was NFSMNT_KERB */
 #define	NFSMNT_DUMBTIMR		0x00000800  /* Don't estimate rtt dynamically */
-/* 0x1000 free, was NFSMNT_LEASETERM */
+#define	NFSMNT_WCOMMITSIZE	0x00001000  /* set max write commit size */
 #define	NFSMNT_READAHEAD	0x00002000  /* set read ahead */
 #define	NFSMNT_DEADTHRESH	0x00004000  /* set dead server retry thresh */
 #define	NFSMNT_RESVPORT		0x00008000  /* Allocate a reserved port */