Re: [PATCH 3/5] vfs: add a zero-initialization mode to fallocate

Eric Biggers
2021-09-20T18:55:49Z

On Fri, Sep 17, 2021 at 06:31:01PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> Add a new mode to fallocate to zero-initialize all the storage backing a
> file.
>
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
> fs/open.c | 5 +++++
> include/linux/falloc.h | 1 +
> include/uapi/linux/falloc.h | 9 +++++++++
> 3 files changed, 15 insertions(+)
>
>
> diff --git a/fs/open.c b/fs/open.c
> index daa324606a41..230220b8f67a 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -256,6 +256,11 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> (mode & ~FALLOC_FL_INSERT_RANGE))
> return -EINVAL;
>
> + /* Zeroinit should only be used by itself and keep size must be set. */
> + if ((mode & FALLOC_FL_ZEROINIT_RANGE) &&
> + (mode != (FALLOC_FL_ZEROINIT_RANGE | FALLOC_FL_KEEP_SIZE)))
> + return -EINVAL;
> +
> /* Unshare range should only be used with allocate mode. */
> if ((mode & FALLOC_FL_UNSHARE_RANGE) &&
> (mode & ~(FALLOC_FL_UNSHARE_RANGE | FALLOC_FL_KEEP_SIZE)))
> diff --git a/include/linux/falloc.h b/include/linux/falloc.h
> index f3f0b97b1675..4597b416667b 100644
> --- a/include/linux/falloc.h
> +++ b/include/linux/falloc.h
> @@ -29,6 +29,7 @@ struct space_resv {
> FALLOC_FL_PUNCH_HOLE | \
> FALLOC_FL_COLLAPSE_RANGE | \
> FALLOC_FL_ZERO_RANGE | \
> + FALLOC_FL_ZEROINIT_RANGE | \
> FALLOC_FL_INSERT_RANGE | \
> FALLOC_FL_UNSHARE_RANGE)
>
> diff --git a/include/uapi/linux/falloc.h b/include/uapi/linux/falloc.h
> index 51398fa57f6c..8144403b6102 100644
> --- a/include/uapi/linux/falloc.h
> +++ b/include/uapi/linux/falloc.h
> @@ -77,4 +77,13 @@
> */
> #define FALLOC_FL_UNSHARE_RANGE 0x40
>
> +/*
> + * FALLOC_FL_ZEROINIT_RANGE is used to reinitialize storage backing a file by
> + * writing zeros to it. Subsequent read and writes should not fail due to any
> + * previous media errors. Blocks must be not be shared or require copy on
> + * write. Holes and unwritten extents are left untouched. This mode must be
> + * used with FALLOC_FL_KEEP_SIZE.
> + */
> +#define FALLOC_FL_ZEROINIT_RANGE 0x80
> +

How does this differ from ZERO_RANGE? Especially when the above says that
ZEROINIT_RANGE leaves unwritten extents untouched. That implies that unwritten
extents are still "allowed". If that's the case, why not just use ZERO_RANGE?

- Eric