[PATCH v3 11/59] netfs: Implement unbuffered/DIO vs buffered I/O locking

David Howells
2023-12-07T21:22:57Z

Borrow NFS's direct-vs-buffered I/O locking into netfslib. Similar code is
also used in ceph.

Modify it to have the correct checker annotations for i_rwsem lock
acquisition/release and to return -ERESTARTSYS if waits are interrupted.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
---
fs/netfs/Makefile | 1 +
fs/netfs/locking.c | 215 ++++++++++++++++++++++++++++++++++++++++++
include/linux/netfs.h | 10 ++
3 files changed, 226 insertions(+)
create mode 100644 fs/netfs/locking.c

diff --git a/fs/netfs/Makefile b/fs/netfs/Makefile
index a84fe9bbd3c4..cf3fc847b8ac 100644
--- a/fs/netfs/Makefile
+++ b/fs/netfs/Makefile
@@ -4,6 +4,7 @@ netfs-y := \
buffered_read.o \
io.o \
iterator.o \
+ locking.o \
main.o \
misc.o \
objects.o
diff --git a/fs/netfs/locking.c b/fs/netfs/locking.c
new file mode 100644
index 000000000000..58e0f48394c5
--- /dev/null
+++ b/fs/netfs/locking.c
@@ -0,0 +1,215 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * I/O and data path helper functionality.
+ *
+ * Borrowed from NFS Copyright (c) 2016 Trond Myklebust
+ */
+
+#include <linux/kernel.h>
+#include <linux/netfs.h>
+
+/*
+ * inode_dio_wait_interruptible - wait for outstanding DIO requests to finish
+ * @inode: inode to wait for
+ *
+ * Waits for all pending direct I/O requests to finish so that we can
+ * proceed with a truncate or equivalent operation.
+ *
+ * Must be called under a lock that serializes taking new references
+ * to i_dio_count, usually by inode->i_mutex.
+ */
+static int inode_dio_wait_interruptible(struct inode *inode)
+{
+ if (!atomic_read(&inode->i_dio_count))
+ return 0;
+
+ wait_queue_head_t *wq = bit_waitqueue(&inode->i_state, __I_DIO_WAKEUP);
+ DEFINE_WAIT_BIT(q, &inode->i_state, __I_DIO_WAKEUP);
+
+ for (;;) {
+ prepare_to_wait(wq, &q.wq_entry, TASK_INTERRUPTIBLE);
+ if (!atomic_read(&inode->i_dio_count))
+ break;
+ if (signal_pending(current))
+ break;
+ schedule();
+ }
+ finish_wait(wq, &q.wq_entry);
+
+ return atomic_read(&inode->i_dio_count) ? -ERESTARTSYS : 0;
+}
+
+/* Call with exclusively locked inode->i_rwsem */
+static int netfs_block_o_direct(struct netfs_inode *ictx)
+{
+ if (!test_bit(NETFS_ICTX_ODIRECT, &ictx->flags))
+ return 0;
+ clear_bit(NETFS_ICTX_ODIRECT, &ictx->flags);
+ return inode_dio_wait_interruptible(&ictx->inode);
+}
+
+/**
+ * netfs_start_io_read - declare the file is being used for buffered reads
+ * @inode: file inode
+ *
+ * Declare that a buffered read operation is about to start, and ensure
+ * that we block all direct I/O.
+ * On exit, the function ensures that the NETFS_ICTX_ODIRECT flag is unset,
+ * and holds a shared lock on inode->i_rwsem to ensure that the flag
+ * cannot be changed.
+ * In practice, this means that buffered read operations are allowed to
+ * execute in parallel, thanks to the shared lock, whereas direct I/O
+ * operations need to wait to grab an exclusive lock in order to set
+ * NETFS_ICTX_ODIRECT.
+ * Note that buffered writes and truncates both take a write lock on
+ * inode->i_rwsem, meaning that those are serialised w.r.t. the reads.
+ */
+int netfs_start_io_read(struct inode *inode)
+ __acquires(inode->i_rwsem)
+{
+ struct netfs_inode *ictx = netfs_inode(inode);
+
+ /* Be an optimist! */
+ if (down_read_interruptible(&inode->i_rwsem) < 0)
+ return -ERESTARTSYS;
+ if (test_bit(NETFS_ICTX_ODIRECT, &ictx->flags) == 0)
+ return 0;
+ up_read(&inode->i_rwsem);
+
+ /* Slow path.... */
+ if (down_write_killable(&inode->i_rwsem) < 0)
+ return -ERESTARTSYS;
+ if (netfs_block_o_direct(ictx) < 0) {
+ up_write(&inode->i_rwsem);
+ return -ERESTARTSYS;
+ }
+ downgrade_write(&inode->i_rwsem);
+ return 0;
+}
+EXPORT_SYMBOL(netfs_start_io_read);
+
+/**
+ * netfs_end_io_read - declare that the buffered read operation is done
+ * @inode: file inode
+ *
+ * Declare that a buffered read operation is done, and release the shared
+ * lock on inode->i_rwsem.
+ */
+void netfs_end_io_read(struct inode *inode)
+ __releases(inode->i_rwsem)
+{
+ up_read(&inode->i_rwsem);
+}
+EXPORT_SYMBOL(netfs_end_io_read);
+
+/**
+ * netfs_start_io_write - declare the file is being used for buffered writes
+ * @inode: file inode
+ *
+ * Declare that a buffered read operation is about to start, and ensure
+ * that we block all direct I/O.
+ */
+int netfs_start_io_write(struct inode *inode)
+ __acquires(inode->i_rwsem)
+{
+ struct netfs_inode *ictx = netfs_inode(inode);
+
+ if (down_write_killable(&inode->i_rwsem) < 0)
+ return -ERESTARTSYS;
+ if (netfs_block_o_direct(ictx) < 0) {
+ up_write(&inode->i_rwsem);
+ return -ERESTARTSYS;
+ }
+ return 0;
+}
+EXPORT_SYMBOL(netfs_start_io_write);
+
+/**
+ * netfs_end_io_write - declare that the buffered write operation is done
+ * @inode: file inode
+ *
+ * Declare that a buffered write operation is done, and release the
+ * lock on inode->i_rwsem.
+ */
+void netfs_end_io_write(struct inode *inode)
+ __releases(inode->i_rwsem)
+{
+ up_write(&inode->i_rwsem);
+}
+EXPORT_SYMBOL(netfs_end_io_write);
+
+/* Call with exclusively locked inode->i_rwsem */
+static int netfs_block_buffered(struct inode *inode)
+{
+ struct netfs_inode *ictx = netfs_inode(inode);
+ int ret;
+
+ if (!test_bit(NETFS_ICTX_ODIRECT, &ictx->flags)) {
+ set_bit(NETFS_ICTX_ODIRECT, &ictx->flags);
+ if (inode->i_mapping->nrpages != 0) {
+ unmap_mapping_range(inode->i_mapping, 0, 0, 0);
+ ret = filemap_fdatawait(inode->i_mapping);
+ if (ret < 0) {
+ clear_bit(NETFS_ICTX_ODIRECT, &ictx->flags);
+ return ret;
+ }
+ }
+ }
+ return 0;
+}
+
+/**
+ * netfs_start_io_direct - declare the file is being used for direct i/o
+ * @inode: file inode
+ *
+ * Declare that a direct I/O operation is about to start, and ensure
+ * that we block all buffered I/O.
+ * On exit, the function ensures that the NETFS_ICTX_ODIRECT flag is set,
+ * and holds a shared lock on inode->i_rwsem to ensure that the flag
+ * cannot be changed.
+ * In practice, this means that direct I/O operations are allowed to
+ * execute in parallel, thanks to the shared lock, whereas buffered I/O
+ * operations need to wait to grab an exclusive lock in order to clear
+ * NETFS_ICTX_ODIRECT.
+ * Note that buffered writes and truncates both take a write lock on
+ * inode->i_rwsem, meaning that those are serialised w.r.t. O_DIRECT.
+ */
+int netfs_start_io_direct(struct inode *inode)
+ __acquires(inode->i_rwsem)
+{
+ struct netfs_inode *ictx = netfs_inode(inode);
+ int ret;
+
+ /* Be an optimist! */
+ if (down_read_interruptible(&inode->i_rwsem) < 0)
+ return -ERESTARTSYS;
+ if (test_bit(NETFS_ICTX_ODIRECT, &ictx->flags) != 0)
+ return 0;
+ up_read(&inode->i_rwsem);
+
+ /* Slow path.... */
+ if (down_write_killable(&inode->i_rwsem) < 0)
+ return -ERESTARTSYS;
+ ret = netfs_block_buffered(inode);
+ if (ret < 0) {
+ up_write(&inode->i_rwsem);
+ return ret;
+ }
+ downgrade_write(&inode->i_rwsem);
+ return 0;
+}
+EXPORT_SYMBOL(netfs_start_io_direct);
+
+/**
+ * netfs_end_io_direct - declare that the direct i/o operation is done
+ * @inode: file inode
+ *
+ * Declare that a direct I/O operation is done, and release the shared
+ * lock on inode->i_rwsem.
+ */
+void netfs_end_io_direct(struct inode *inode)
+ __releases(inode->i_rwsem)
+{
+ up_read(&inode->i_rwsem);
+}
+EXPORT_SYMBOL(netfs_end_io_direct);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 8efbfd3b2820..fc6d9756a029 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -129,6 +129,8 @@ struct netfs_inode {
struct fscache_cookie *cache;
#endif
loff_t remote_i_size; /* Size of the remote file */
+ unsigned long flags;
+#define NETFS_ICTX_ODIRECT 0 /* The file has DIO in progress */
};

/*
@@ -310,6 +312,13 @@ ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
struct iov_iter *new,
iov_iter_extraction_t extraction_flags);

+int netfs_start_io_read(struct inode *inode);
+void netfs_end_io_read(struct inode *inode);
+int netfs_start_io_write(struct inode *inode);
+void netfs_end_io_write(struct inode *inode);
+int netfs_start_io_direct(struct inode *inode);
+void netfs_end_io_direct(struct inode *inode);
+
/**
* netfs_inode - Get the netfs inode context from the inode
* @inode: The inode to query
@@ -335,6 +344,7 @@ static inline void netfs_inode_init(struct netfs_inode *ctx,
{
ctx->ops = ops;
ctx->remote_i_size = i_size_read(&ctx->inode);
+ ctx->flags = 0;
#if IS_ENABLED(CONFIG_FSCACHE)
ctx->cache = NULL;
#endif