nfs: Provide a splice-read wrapper

Provide a splice_read wrapper for NFS.  This locks the inode around
filemap_splice_read() and revalidates the mapping.  Splicing from direct
I/O is handled by the caller.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: Trond Myklebust <trond.myklebust@hammerspace.com>
cc: Anna Schumaker <anna@kernel.org>
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/20230522135018.2742245-21-dhowells@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/nfs/file.c

@@ -178,6 +178,27 @@ nfs_file_read(struct kiocb *iocb, struct iov_iter *to)
 }
 EXPORT_SYMBOL_GPL(nfs_file_read);
 
+ssize_t
+nfs_file_splice_read(struct file *in, loff_t *ppos, struct pipe_inode_info *pipe,
+		     size_t len, unsigned int flags)
+{
+	struct inode *inode = file_inode(in);
+	ssize_t result;
+
+	dprintk("NFS: splice_read(%pD2, %zu@%llu)\n", in, len, *ppos);
+
+	nfs_start_io_read(inode);
+	result = nfs_revalidate_mapping(inode, in->f_mapping);
+	if (!result) {
+		result = filemap_splice_read(in, ppos, pipe, len, flags);
+		if (result > 0)
+			nfs_add_stats(inode, NFSIOS_NORMALREADBYTES, result);
+	}
+	nfs_end_io_read(inode);
+	return result;
+}
+EXPORT_SYMBOL_GPL(nfs_file_splice_read);
+
 int
 nfs_file_mmap(struct file * file, struct vm_area_struct * vma)
 {
@@ -879,7 +900,7 @@ const struct file_operations nfs_file_operations = {
 	.fsync		= nfs_file_fsync,
 	.lock		= nfs_lock,
 	.flock		= nfs_flock,
-	.splice_read	= generic_file_splice_read,
+	.splice_read	= nfs_file_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.check_flags	= nfs_check_flags,
 	.setlease	= simple_nosetlease,

fs/nfs/internal.h

@@ -416,6 +416,8 @@ static inline __u32 nfs_access_xattr_mask(const struct nfs_server *server)
 int nfs_file_fsync(struct file *file, loff_t start, loff_t end, int datasync);
 loff_t nfs_file_llseek(struct file *, loff_t, int);
 ssize_t nfs_file_read(struct kiocb *, struct iov_iter *);
+ssize_t nfs_file_splice_read(struct file *in, loff_t *ppos, struct pipe_inode_info *pipe,
+			     size_t len, unsigned int flags);
 int nfs_file_mmap(struct file *, struct vm_area_struct *);
 ssize_t nfs_file_write(struct kiocb *, struct iov_iter *);
 int nfs_file_release(struct inode *, struct file *);

fs/nfs/nfs4file.c

@@ -454,7 +454,7 @@ const struct file_operations nfs4_file_operations = {
 	.fsync		= nfs_file_fsync,
 	.lock		= nfs_lock,
 	.flock		= nfs_flock,
-	.splice_read	= generic_file_splice_read,
+	.splice_read	= nfs_file_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.check_flags	= nfs_check_flags,
 	.setlease	= nfs4_setlease,