[PATCH v2] NFS: Fix fscache read from NFS after cache error

[Dave Wysochanski <dwysocha@redhat.com>]

Earlier commits refactored some NFS read code and removed
nfs_readpage_async(), but neglected to properly fixup
nfs_readpage_from_fscache_complete().  The code path is
only hit when something unusual occurs with the cachefiles
backing filesystem, such as an IO error or while a cookie
is being invalidated.

Fixup this path by unconditionally unlocking the page
and letting the VM decide what to do based on PG_uptodate.
Note that the VM re-issue of the IO will likely go back
to fscache, so we may end up failing the read if fscache
has a permanent error (such as EIO).

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
---
 fs/nfs/fscache.c | 7 +++----
 fs/nfs/read.c    | 5 +++--
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/nfs/fscache.c b/fs/nfs/fscache.c
index c4c021c6ebbd..dca7ce676b1d 100644
--- a/fs/nfs/fscache.c
+++ b/fs/nfs/fscache.c
@@ -385,12 +385,11 @@ static void nfs_readpage_from_fscache_complete(struct page *page,
 		 "NFS: readpage_from_fscache_complete (0x%p/0x%p/%d)\n",
 		 page, context, error);
 
-	/* if the read completes with an error, we just unlock the page and let
+	/* if the read completes with an error, unlock the page and let
 	 * the VM reissue the readpage */
-	if (!error) {
+	if (!error)
 		SetPageUptodate(page);
-		unlock_page(page);
-	}
+	unlock_page(page);
 }
 
 /*
diff --git a/fs/nfs/read.c b/fs/nfs/read.c
index eb390eb618b3..9f39e0a1a38b 100644
--- a/fs/nfs/read.c
+++ b/fs/nfs/read.c
@@ -362,13 +362,13 @@ int nfs_readpage(struct file *file, struct page *page)
 	} else
 		desc.ctx = get_nfs_open_context(nfs_file_open_context(file));
 
+	xchg(&desc.ctx->error, 0);
 	if (!IS_SYNC(inode)) {
 		ret = nfs_readpage_from_fscache(desc.ctx, inode, page);
 		if (ret == 0)
-			goto out;
+			goto out_wait;
 	}
 
-	xchg(&desc.ctx->error, 0);
 	nfs_pageio_init_read(&desc.pgio, inode, false,
 			     &nfs_async_read_completion_ops);
 
@@ -378,6 +378,7 @@ int nfs_readpage(struct file *file, struct page *page)
 
 	nfs_pageio_complete_read(&desc.pgio);
 	ret = desc.pgio.pg_error < 0 ? desc.pgio.pg_error : 0;
+out_wait:
 	if (!ret) {
 		ret = wait_on_page_locked_killable(page);
 		if (!PageUptodate(page) && !ret)
-- 
1.8.3.1