Subject: Re: [PATCH v4 11/39] netfs: Implement unbuffered/DIO vs buffered I/O locking

From: Jeff Layton <[email-masked]>
Date: Wed, 13 Dec 2023 11:08:23 -0500

On Wed, 2023-12-13 at 15:23 +0000, David Howells wrote:
> 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.
> 

This is just adding new infrastructure. It'd be nice to go ahead and
convert a filesystem to use this at the same time. Ceph would be a good
candidate. Otherwise, I'm not sure how this shakes out as far as
cleanliness in the callers.

> Signed-off-by: David Howells <[email-masked]>
> cc: Jeff Layton <[email-masked]>
> cc: [email-masked]
> cc: [email-masked]
> cc: [email-masked]
> ---
>  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
> 
-- 
Jeff Layton <[email-masked]>