Re: [PATCH v8 03/10] fs: Initial atomic write support

[Darrick J. Wong]

On Mon, Jun 10, 2024 at 10:43:22AM +0000, John Garry wrote:
> From: Prasad Singamsetty <prasad.singamsetty@xxxxxxxxxx>
> 
> An atomic write is a write issued with torn-write protection, meaning
> that for a power failure or any other hardware failure, all or none of the
> data from the write will be stored, but never a mix of old and new data.
> 
> Userspace may add flag RWF_ATOMIC to pwritev2() to indicate that the
> write is to be issued with torn-write prevention, according to special
> alignment and length rules.
> 
> For any syscall interface utilizing struct iocb, add IOCB_ATOMIC for
> iocb->ki_flags field to indicate the same.
> 
> A call to statx will give the relevant atomic write info for a file:
> - atomic_write_unit_min
> - atomic_write_unit_max
> - atomic_write_segments_max
> 
> Both min and max values must be a power-of-2.
> 
> Applications can avail of atomic write feature by ensuring that the total
> length of a write is a power-of-2 in size and also sized between
> atomic_write_unit_min and atomic_write_unit_max, inclusive. Applications
> must ensure that the write is at a naturally-aligned offset in the file
> wrt the total write length. The value in atomic_write_segments_max
> indicates the upper limit for IOV_ITER iovcnt.
> 
> Add file mode flag FMODE_CAN_ATOMIC_WRITE, so files which do not have the
> flag set will have RWF_ATOMIC rejected and not just ignored.
> 
> Add a type argument to kiocb_set_rw_flags() to allows reads which have
> RWF_ATOMIC set to be rejected.
> 
> Helper function generic_atomic_write_valid() can be used by FSes to verify
> compliant writes. There we check for iov_iter type is for ubuf, which
> implies iovcnt==1 for pwritev2(), which is an initial restriction for
> atomic_write_segments_max. Initially the only user will be bdev file
> operations write handler. We will rely on the block BIO submission path to
> ensure write sizes are compliant for the bdev, so we don't need to check
> atomic writes sizes yet.
> 
> Signed-off-by: Prasad Singamsetty <prasad.singamsetty@xxxxxxxxxx>
> jpg: merge into single patch and much rewrite
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>

Seems fine to me, though clearly others have had much stronger opinions
in the past so:
Acked-by: Darrick J. Wong <djwong@xxxxxxxxxx>

--D

> ---
>  fs/aio.c                |  8 ++++----
>  fs/btrfs/ioctl.c        |  2 +-
>  fs/read_write.c         | 18 +++++++++++++++++-
>  include/linux/fs.h      | 17 +++++++++++++++--
>  include/uapi/linux/fs.h |  5 ++++-
>  io_uring/rw.c           |  9 ++++-----
>  6 files changed, 45 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 57c9f7c077e6..93ef59d358b3 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1516,7 +1516,7 @@ static void aio_complete_rw(struct kiocb *kiocb, long res)
>  	iocb_put(iocb);
>  }
>  
> -static int aio_prep_rw(struct kiocb *req, const struct iocb *iocb)
> +static int aio_prep_rw(struct kiocb *req, const struct iocb *iocb, int rw_type)
>  {
>  	int ret;
>  
> @@ -1542,7 +1542,7 @@ static int aio_prep_rw(struct kiocb *req, const struct iocb *iocb)
>  	} else
>  		req->ki_ioprio = get_current_ioprio();
>  
> -	ret = kiocb_set_rw_flags(req, iocb->aio_rw_flags);
> +	ret = kiocb_set_rw_flags(req, iocb->aio_rw_flags, rw_type);
>  	if (unlikely(ret))
>  		return ret;
>  
> @@ -1594,7 +1594,7 @@ static int aio_read(struct kiocb *req, const struct iocb *iocb,
>  	struct file *file;
>  	int ret;
>  
> -	ret = aio_prep_rw(req, iocb);
> +	ret = aio_prep_rw(req, iocb, READ);
>  	if (ret)
>  		return ret;
>  	file = req->ki_filp;
> @@ -1621,7 +1621,7 @@ static int aio_write(struct kiocb *req, const struct iocb *iocb,
>  	struct file *file;
>  	int ret;
>  
> -	ret = aio_prep_rw(req, iocb);
> +	ret = aio_prep_rw(req, iocb, WRITE);
>  	if (ret)
>  		return ret;
>  	file = req->ki_filp;
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index efd5d6e9589e..6ad524b894fc 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -4627,7 +4627,7 @@ static int btrfs_ioctl_encoded_write(struct file *file, void __user *argp, bool
>  		goto out_iov;
>  
>  	init_sync_kiocb(&kiocb, file);
> -	ret = kiocb_set_rw_flags(&kiocb, 0);
> +	ret = kiocb_set_rw_flags(&kiocb, 0, WRITE);
>  	if (ret)
>  		goto out_iov;
>  	kiocb.ki_pos = pos;
> diff --git a/fs/read_write.c b/fs/read_write.c
> index ef6339391351..285b0f5a9a9c 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -730,7 +730,7 @@ static ssize_t do_iter_readv_writev(struct file *filp, struct iov_iter *iter,
>  	ssize_t ret;
>  
>  	init_sync_kiocb(&kiocb, filp);
> -	ret = kiocb_set_rw_flags(&kiocb, flags);
> +	ret = kiocb_set_rw_flags(&kiocb, flags, type);
>  	if (ret)
>  		return ret;
>  	kiocb.ki_pos = (ppos ? *ppos : 0);
> @@ -1736,3 +1736,19 @@ int generic_file_rw_checks(struct file *file_in, struct file *file_out)
>  
>  	return 0;
>  }
> +
> +bool generic_atomic_write_valid(struct iov_iter *iter, loff_t pos)
> +{
> +	size_t len = iov_iter_count(iter);
> +
> +	if (!iter_is_ubuf(iter))
> +		return false;
> +
> +	if (!is_power_of_2(len))
> +		return false;
> +
> +	if (!IS_ALIGNED(pos, len))
> +		return false;
> +
> +	return true;
> +}
> \ No newline at end of file
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 0283cf366c2a..e049414bef7d 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -125,8 +125,10 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
>  #define FMODE_EXEC		((__force fmode_t)(1 << 5))
>  /* File writes are restricted (block device specific) */
>  #define FMODE_WRITE_RESTRICTED	((__force fmode_t)(1 << 6))
> +/* File supports atomic writes */
> +#define FMODE_CAN_ATOMIC_WRITE	((__force fmode_t)(1 << 7))
>  
> -/* FMODE_* bits 7 to 8 */
> +/* FMODE_* bit 8 */
>  
>  /* 32bit hashes as llseek() offset (for directories) */
>  #define FMODE_32BITHASH         ((__force fmode_t)(1 << 9))
> @@ -317,6 +319,7 @@ struct readahead_control;
>  #define IOCB_SYNC		(__force int) RWF_SYNC
>  #define IOCB_NOWAIT		(__force int) RWF_NOWAIT
>  #define IOCB_APPEND		(__force int) RWF_APPEND
> +#define IOCB_ATOMIC		(__force int) RWF_ATOMIC
>  
>  /* non-RWF related bits - start at 16 */
>  #define IOCB_EVENTFD		(1 << 16)
> @@ -351,6 +354,7 @@ struct readahead_control;
>  	{ IOCB_SYNC,		"SYNC" }, \
>  	{ IOCB_NOWAIT,		"NOWAIT" }, \
>  	{ IOCB_APPEND,		"APPEND" }, \
> +	{ IOCB_ATOMIC,		"ATOMIC"}, \
>  	{ IOCB_EVENTFD,		"EVENTFD"}, \
>  	{ IOCB_DIRECT,		"DIRECT" }, \
>  	{ IOCB_WRITE,		"WRITE" }, \
> @@ -3403,7 +3407,8 @@ static inline int iocb_flags(struct file *file)
>  	return res;
>  }
>  
> -static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
> +static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags,
> +				     int rw_type)
>  {
>  	int kiocb_flags = 0;
>  
> @@ -3422,6 +3427,12 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
>  			return -EOPNOTSUPP;
>  		kiocb_flags |= IOCB_NOIO;
>  	}
> +	if (flags & RWF_ATOMIC) {
> +		if (rw_type != WRITE)
> +			return -EOPNOTSUPP;
> +		if (!(ki->ki_filp->f_mode & FMODE_CAN_ATOMIC_WRITE))
> +			return -EOPNOTSUPP;
> +	}
>  	kiocb_flags |= (__force int) (flags & RWF_SUPPORTED);
>  	if (flags & RWF_SYNC)
>  		kiocb_flags |= IOCB_DSYNC;
> @@ -3613,4 +3624,6 @@ extern int vfs_fadvise(struct file *file, loff_t offset, loff_t len,
>  extern int generic_fadvise(struct file *file, loff_t offset, loff_t len,
>  			   int advice);
>  
> +bool generic_atomic_write_valid(struct iov_iter *iter, loff_t pos);
> +
>  #endif /* _LINUX_FS_H */
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 45e4e64fd664..191a7e88a8ab 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -329,9 +329,12 @@ typedef int __bitwise __kernel_rwf_t;
>  /* per-IO negation of O_APPEND */
>  #define RWF_NOAPPEND	((__force __kernel_rwf_t)0x00000020)
>  
> +/* Atomic Write */
> +#define RWF_ATOMIC	((__force __kernel_rwf_t)0x00000040)
> +
>  /* mask of flags supported by the kernel */
>  #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
> -			 RWF_APPEND | RWF_NOAPPEND)
> +			 RWF_APPEND | RWF_NOAPPEND | RWF_ATOMIC)
>  
>  /* Pagemap ioctl */
>  #define PAGEMAP_SCAN	_IOWR('f', 16, struct pm_scan_arg)
> diff --git a/io_uring/rw.c b/io_uring/rw.c
> index 1a2128459cb4..c004d21e2f12 100644
> --- a/io_uring/rw.c
> +++ b/io_uring/rw.c
> @@ -772,7 +772,7 @@ static bool need_complete_io(struct io_kiocb *req)
>  		S_ISBLK(file_inode(req->file)->i_mode);
>  }
>  
> -static int io_rw_init_file(struct io_kiocb *req, fmode_t mode)
> +static int io_rw_init_file(struct io_kiocb *req, fmode_t mode, int rw_type)
>  {
>  	struct io_rw *rw = io_kiocb_to_cmd(req, struct io_rw);
>  	struct kiocb *kiocb = &rw->kiocb;
> @@ -787,7 +787,7 @@ static int io_rw_init_file(struct io_kiocb *req, fmode_t mode)
>  		req->flags |= io_file_get_flags(file);
>  
>  	kiocb->ki_flags = file->f_iocb_flags;
> -	ret = kiocb_set_rw_flags(kiocb, rw->flags);
> +	ret = kiocb_set_rw_flags(kiocb, rw->flags, rw_type);
>  	if (unlikely(ret))
>  		return ret;
>  	kiocb->ki_flags |= IOCB_ALLOC_CACHE;
> @@ -832,8 +832,7 @@ static int __io_read(struct io_kiocb *req, unsigned int issue_flags)
>  		if (unlikely(ret < 0))
>  			return ret;
>  	}
> -
> -	ret = io_rw_init_file(req, FMODE_READ);
> +	ret = io_rw_init_file(req, FMODE_READ, READ);
>  	if (unlikely(ret))
>  		return ret;
>  	req->cqe.res = iov_iter_count(&io->iter);
> @@ -1013,7 +1012,7 @@ int io_write(struct io_kiocb *req, unsigned int issue_flags)
>  	ssize_t ret, ret2;
>  	loff_t *ppos;
>  
> -	ret = io_rw_init_file(req, FMODE_WRITE);
> +	ret = io_rw_init_file(req, FMODE_WRITE, WRITE);
>  	if (unlikely(ret))
>  		return ret;
>  	req->cqe.res = iov_iter_count(&io->iter);
> -- 
> 2.31.1
> 
>