[PATCH v13 05/12] famfs: Introduce file_operations read/write

[John Groves]

From: John Groves <john@xxxxxxxxxx>

This commit introduces fs/famfs/famfs_file.c and the famfs
file_operations for read/write.

This is not usable yet because:

* It calls dax_iomap_rw() with NULL iomap_ops (which will be
  introduced in a subsequent commit).
* famfs_ioctl() is coming in a later commit, and it is necessary
  to map a file to a memory allocation.

Signed-off-by: John Groves <john@xxxxxxxxxx>
---
v13:
 - Use copy_splice_read() instead of filemap_splice_read() for .splice_read:
   famfs inodes are DAX (no page cache), so filemap_splice_read() faulted in
   zero-filled ram_aops folios and splice()/sendfile() returned a stream of
   zeroes; copy_splice_read() routes through ->read_iter (dax_iomap_rw) and
   returns the real data (Sashiko bot).
 - No code change. The Sashiko bot flagged that famfs_dax_write_iter() does
   not call generic_write_sync() for O_SYNC/O_DSYNC writes. Considered and
   declined -- the finding does not apply to famfs: there is no page cache,
   no dirty tracking and no writeback, and ->fsync is noop_fsync by design,
   so generic_write_sync() resolves to vfs_fsync_range() -> noop_fsync() -> 0,
   an inert no-op. Bla bla bla ;)

 fs/famfs/Makefile         |   2 +-
 fs/famfs/famfs_file.c     | 141 ++++++++++++++++++++++++++++++++++++++
 fs/famfs/famfs_inode.c    |   2 +-
 fs/famfs/famfs_internal.h |   2 +
 4 files changed, 145 insertions(+), 2 deletions(-)
 create mode 100644 fs/famfs/famfs_file.c

diff --git a/fs/famfs/Makefile b/fs/famfs/Makefile
index 62230bcd6793..8cac90c090a4 100644
--- a/fs/famfs/Makefile
+++ b/fs/famfs/Makefile
@@ -2,4 +2,4 @@
 
 obj-$(CONFIG_FAMFS) += famfs.o
 
-famfs-y := famfs_inode.o
+famfs-y := famfs_inode.o famfs_file.o
diff --git a/fs/famfs/famfs_file.c b/fs/famfs/famfs_file.c
new file mode 100644
index 000000000000..1369fe1824bc
--- /dev/null
+++ b/fs/famfs/famfs_file.c
@@ -0,0 +1,141 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * famfs - dax file system for shared fabric-attached memory
+ *
+ * Copyright 2023-2024 Micron Technology, Inc.
+ *
+ * This file system, originally based on ramfs the dax support from xfs,
+ * is intended to allow multiple host systems to mount a common file system
+ * view of dax files that map to shared memory.
+ */
+
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/dax.h>
+#include <linux/iomap.h>
+
+#include "famfs_internal.h"
+
+/*********************************************************************
+ * file_operations
+ */
+
+/* Reject I/O to files that aren't in a valid state */
+static ssize_t
+famfs_file_invalid(struct inode *inode)
+{
+	if (!IS_DAX(inode)) {
+		pr_debug("%s: inode %llx IS_DAX is false\n",
+			 __func__, (u64)inode);
+		return -ENXIO;
+	}
+	return 0;
+}
+
+static ssize_t
+famfs_rw_prep(struct kiocb *iocb, struct iov_iter *ubuf)
+{
+	struct inode *inode = iocb->ki_filp->f_mapping->host;
+	struct super_block *sb = inode->i_sb;
+	struct famfs_fs_info *fsi = sb->s_fs_info;
+	size_t i_size = i_size_read(inode);
+	size_t count = iov_iter_count(ubuf);
+	size_t max_count;
+	ssize_t rc;
+
+	if (fsi->deverror)
+		return -ENODEV;
+
+	rc = famfs_file_invalid(inode);
+	if (rc)
+		return rc;
+
+	/* Avoid unsigned underflow if position is past EOF */
+	if (iocb->ki_pos >= i_size)
+		max_count = 0;
+	else
+		max_count = i_size - iocb->ki_pos;
+
+	if (count > max_count)
+		iov_iter_truncate(ubuf, max_count);
+
+	if (!iov_iter_count(ubuf))
+		return 0;
+
+	return rc;
+}
+
+static ssize_t
+famfs_dax_read_iter(struct kiocb *iocb, struct iov_iter	*to)
+{
+	struct inode *inode = iocb->ki_filp->f_mapping->host;
+	ssize_t rc;
+
+	/* dax_iomap_rw() requires i_rwsem held (shared for read) */
+	inode_lock_shared(inode);
+	rc = famfs_rw_prep(iocb, to);
+	if (rc || !iov_iter_count(to)) {
+		inode_unlock_shared(inode);
+		return rc;
+	}
+
+	rc = dax_iomap_rw(iocb, to, NULL /*&famfs_iomap_ops */);
+	inode_unlock_shared(inode);
+
+	if (rc > 0)
+		file_accessed(iocb->ki_filp);
+	return rc;
+}
+
+/**
+ * famfs_dax_write_iter()
+ *
+ * We need our own write-iter in order to prevent append
+ *
+ * @iocb:
+ * @from: iterator describing the user memory source for the write
+ */
+static ssize_t
+famfs_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
+{
+	struct inode *inode = iocb->ki_filp->f_mapping->host;
+	struct famfs_fs_info *fsi = inode->i_sb->s_fs_info;
+	ssize_t rc;
+
+	if (!famfs_opt_enabled(fsi, FAMFS_OPT_WRITE))
+		return -EPERM;
+
+	/* dax_iomap_rw() requires i_rwsem held (exclusive for write) */
+	inode_lock(inode);
+	rc = famfs_rw_prep(iocb, from);
+	if (rc || !iov_iter_count(from)) {
+		inode_unlock(inode);
+		return rc;
+	}
+
+	kiocb_modified(iocb); /* mtime/ctime + strip set[e]uid */
+
+	rc = dax_iomap_rw(iocb, from, NULL /*&famfs_iomap_ops*/);
+	inode_unlock(inode);
+	return rc;
+}
+
+const struct file_operations famfs_file_operations = {
+	.owner             = THIS_MODULE,
+
+	/* Custom famfs operations */
+	.write_iter	   = famfs_dax_write_iter,
+	.read_iter	   = famfs_dax_read_iter,
+	.unlocked_ioctl    = NULL /*famfs_file_ioctl*/,
+	.mmap		   = NULL /* famfs_file_mmap */,
+
+	/* Force PMD alignment for mmap */
+	.get_unmapped_area = thp_get_unmapped_area,
+
+	/* Generic Operations */
+	.fsync		   = noop_fsync,
+	.splice_read	   = copy_splice_read,
+	.splice_write	   = iter_file_splice_write,
+	.llseek		   = generic_file_llseek,
+};
+
diff --git a/fs/famfs/famfs_inode.c b/fs/famfs/famfs_inode.c
index b7a3d8d6ee8c..9e8662c4ac98 100644
--- a/fs/famfs/famfs_inode.c
+++ b/fs/famfs/famfs_inode.c
@@ -59,7 +59,7 @@ famfs_get_inode(
 		break;
 	case S_IFREG:
 		inode->i_op = &famfs_file_inode_operations;
-		inode->i_fop = NULL /* &famfs_file_operations */;
+		inode->i_fop = &famfs_file_operations;
 		break;
 	case S_IFDIR:
 		inode->i_op = &famfs_dir_inode_operations;
diff --git a/fs/famfs/famfs_internal.h b/fs/famfs/famfs_internal.h
index 30f0b01010d5..ff9f1d3f686e 100644
--- a/fs/famfs/famfs_internal.h
+++ b/fs/famfs/famfs_internal.h
@@ -15,6 +15,8 @@
 #include <linux/bits.h>
 #include <linux/build_bug.h>
 
+extern const struct file_operations famfs_file_operations;
+
 struct famfs_mount_opts {
 	umode_t mode;
 };
-- 
2.53.0