A problem with the old NFS client where large writes to large files

would sometimes result in a corrupted file was reported via email.
This problem appears to have been caused by r251719 (reverting
r251719 fixed the problem). Although I have not been able to
reproduce this problem, I suspect it is caused by another thread
increasing np->n_size after the mtx_unlock(&np->n_mtx) but before
the vnode_pager_setsize() call. Since the np->n_mtx mutex serializes
updates to np->n_size, doing the vnode_pager_setsize() with the
mutex locked appears to avoid the problem.
Unfortunately, vnode_pager_setsize() where the new size is smaller,
cannot be called with a mutex held.
This patch returns the semantics to be close to pre-r251719 such that the
call to the vnode_pager_setsize() is only delayed until after the mutex is
unlocked when np->n_size is shrinking. Since the file is growing
when being written, I believe this will fix the corruption.

Reported by:	David G. Lawrence (dg@dglawrence.com)
Tested by:	David G. Lawrence (pending, to happen soon)
Reviewed by:	kib
MFC after:	1 week

1 files changed, 13 insertions, 2 deletions

diff --git a/sys/nfsclient/nfs_subs.c b/sys/nfsclient/nfs_subs.c
index c41bc61..5dfab1c 100644
--- a/sys/nfsclient/nfs_subs.c
+++ b/sys/nfsclient/nfs_subs.c
@@ -581,6 +581,7 @@ nfs_loadattrcache(struct vnode **vpp, struct mbuf **mdp, caddr_t *dposp,
 				vap->va_size = np->n_size;
 				np->n_attrstamp = 0;
 				KDTRACE_NFS_ATTRCACHE_FLUSH_DONE(vp);
+				vnode_pager_setsize(vp, np->n_size);
 			} else if (np->n_flag & NMODIFIED) {
 				/*
 				 * We've modified the file: Use the larger
@@ -592,12 +593,22 @@ nfs_loadattrcache(struct vnode **vpp, struct mbuf **mdp, caddr_t *dposp,
 					np->n_size = vap->va_size;
 					np->n_flag |= NSIZECHANGED;
 				}
+				vnode_pager_setsize(vp, np->n_size);
+			} else if (vap->va_size < np->n_size) {
+				/*
+				 * When shrinking the size, the call to
+				 * vnode_pager_setsize() cannot be done
+				 * with the mutex held, so delay it until
+				 * after the mtx_unlock call.
+				 */
+				nsize = np->n_size = vap->va_size;
+				np->n_flag |= NSIZECHANGED;
+				setnsize = 1;
 			} else {
 				np->n_size = vap->va_size;
 				np->n_flag |= NSIZECHANGED;
+				vnode_pager_setsize(vp, np->n_size);
 			}
-			setnsize = 1;
-			nsize = vap->va_size;
 		} else {
 			np->n_size = vap->va_size;
 		}