Re: [PATCH v4 03/11] fs: Initial atomic write support

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

John Garry <john.g.garry@oracle.com> writes:

> From: Prasad Singamsetty <prasad.singamsetty@oracle.com>
>
> 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 atomic_write_valid() can be used by FSes to verify
> compliant writes.
>
> Signed-off-by: Prasad Singamsetty <prasad.singamsetty@oracle.com>
> #jpg: merge into single patch and much rewrite

^^^ this might be a miss I guess.

> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  fs/aio.c                |  8 ++++----
>  fs/btrfs/ioctl.c        |  2 +-
>  fs/read_write.c         |  2 +-
>  include/linux/fs.h      | 36 +++++++++++++++++++++++++++++++++++-
>  include/uapi/linux/fs.h |  5 ++++-
>  io_uring/rw.c           |  4 ++--
>  6 files changed, 47 insertions(+), 10 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index bb2ff48991f3..21bcbc076fd0 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1502,7 +1502,7 @@ 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 type)

maybe rw_type?

>  {
>  	int ret;
>  
> @@ -1528,7 +1528,7 @@ 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, type);
>  	if (unlikely(ret))
>  		return ret;
>  
> @@ -1580,7 +1580,7 @@ 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;
> @@ -1607,7 +1607,7 @@ 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 ac3316e0d11c..455f06d94b11 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -4555,7 +4555,7 @@ 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 d4c036e82b6c..a7dc1819192d 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);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 023f37c60709..7271640fd600 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -43,6 +43,7 @@
>  #include <linux/cred.h>
>  #include <linux/mnt_idmapping.h>
>  #include <linux/slab.h>
> +#include <linux/uio.h>
>  
>  #include <asm/byteorder.h>
>  #include <uapi/linux/fs.h>
> @@ -119,6 +120,10 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
>  #define FMODE_PWRITE		((__force fmode_t)0x10)
>  /* File is opened for execution with sys_execve / sys_uselib */
>  #define FMODE_EXEC		((__force fmode_t)0x20)
> +
> +/* File supports atomic writes */
> +#define FMODE_CAN_ATOMIC_WRITE	((__force fmode_t)0x40)
> +
>  /* 32bit hashes as llseek() offset (for directories) */
>  #define FMODE_32BITHASH         ((__force fmode_t)0x200)
>  /* 64bit hashes as llseek() offset (for directories) */
> @@ -328,6 +333,7 @@ enum rw_hint {
>  #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
>  

You might also want to add this definition in here too

#define TRACE_IOCB_STRINGS \
<...>
<...>
{ IOCB_ATOMIC, "ATOMIC" }


>  /* non-RWF related bits - start at 16 */
>  #define IOCB_EVENTFD		(1 << 16)
> @@ -3321,7 +3327,7 @@ 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 type)

maybe rw_type? 

>  {
>  	int kiocb_flags = 0;
>  
> @@ -3338,6 +3344,12 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
>  			return -EOPNOTSUPP;
>  		kiocb_flags |= IOCB_NOIO;
>  	}
> +	if (flags & RWF_ATOMIC) {
> +		if (type == READ)
> +			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;
> @@ -3523,4 +3535,26 @@ 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);
>  
> +static inline bool atomic_write_valid(loff_t pos, struct iov_iter *iter,
> +			   unsigned int unit_min, unsigned int unit_max)
> +{
> +	size_t len = iov_iter_count(iter);
> +
> +	if (!iter_is_ubuf(iter))
> +		return false;

There is no mention about this limitation in the commit message of this
patch. Maybe it will be good to capture why this limitation to only
support ubuf and/or any plans to lift this restriction in future
in the commit message?


> +
> +	if (len == unit_min || len == unit_max) {
> +		/* ok if exactly min or max */
> +	} else if (len < unit_min || len > unit_max) {
> +		return false;
> +	} else if (!is_power_of_2(len)) {
> +		return false;
> +	}

Checking for len == unit_min || len == unit_max is redundant when
unit_min and unit_max are already power of 2.


> +
> +	if (pos & (len - 1))
> +		return false;
> +
> +	return true;
> +}
> +
>  #endif /* _LINUX_FS_H */
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 48ad69f7722e..a0975ae81e64 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -301,9 +301,12 @@ typedef int __bitwise __kernel_rwf_t;
>  /* per-IO O_APPEND */
>  #define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)
>  
> +/* 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_APPEND | 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 d5e79d9bdc71..f8c022301cf4 100644
> --- a/io_uring/rw.c
> +++ b/io_uring/rw.c
> @@ -719,7 +719,7 @@ static int io_rw_init_file(struct io_kiocb *req, fmode_t mode)
>  	struct kiocb *kiocb = &rw->kiocb;
>  	struct io_ring_ctx *ctx = req->ctx;
>  	struct file *file = req->file;
> -	int ret;
> +	int ret, type = (mode == FMODE_WRITE) ? WRITE : READ;
>  
>  	if (unlikely(!file || !(file->f_mode & mode)))
>  		return -EBADF;
> @@ -728,7 +728,7 @@ 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, type);
>  	if (unlikely(ret))
>  		return ret;
>  	kiocb->ki_flags |= IOCB_ALLOC_CACHE;
> -- 
> 2.31.1