Re: [PATCH V2] libxfs: use FALLOC_FL_ZERO_RANGE in libxfs_device_zero

Dave Chinner
2020-02-14T01:35:07Z

On Thu, Feb 13, 2020 at 07:05:50PM -0600, Eric Sandeen wrote:
> I had a request from someone who cared about mkfs speed(!)
> over a slower network block device to look into using faster
> zeroing methods, particularly for the log, during mkfs.
>
> Using FALLOC_FL_ZERO_RANGE is faster in this case than writing
> a bunch of zeros across a wire.
>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>
> V2: Clean up all the nasty stuff I'd flung out there as a wild first
> cut, thanks Dave.
>
> diff --git a/include/builddefs.in b/include/builddefs.in
> index 4700b527..1dd27f76 100644
> --- a/include/builddefs.in
> +++ b/include/builddefs.in
> @@ -144,6 +144,9 @@ endif
> ifeq ($(HAVE_GETFSMAP),yes)
> PCFLAGS+= -DHAVE_GETFSMAP
> endif
> +ifeq ($(HAVE_FALLOCATE),yes)
> +PCFLAGS += -DHAVE_FALLOCATE
> +endif
>
> LIBICU_LIBS = @libicu_LIBS@
> LIBICU_CFLAGS = @libicu_CFLAGS@
> diff --git a/include/linux.h b/include/linux.h
> index 8f3c32b0..8d5c4584 100644
> --- a/include/linux.h
> +++ b/include/linux.h
> @@ -20,6 +20,10 @@
> #include <stdio.h>
> #include <asm/types.h>
> #include <mntent.h>
> +#include <fcntl.h>
> +#if defined(HAVE_FALLOCATE)
> +#include <linux/falloc.h>
> +#endif
> #ifdef OVERRIDE_SYSTEM_FSXATTR
> # define fsxattr sys_fsxattr
> #endif
> @@ -164,6 +168,24 @@ static inline void platform_mntent_close(struct mntent_cursor * cursor)
> endmntent(cursor->mtabp);
> }
>
> +#if defined(FALLOC_FL_ZERO_RANGE)
> +static inline int
> +platform_zero_range(
> + int fd,
> + xfs_off_t start,
> + size_t len)
> +{
> + int ret;
> +
> + ret = fallocate(fd, FALLOC_FL_ZERO_RANGE, start, len);
> + if (!ret)
> + return 0;
> + return -errno;
> +}
> +#else
> +#define platform_zero_range(fd, s, l) (-EOPNOTSUPP)
> +#endif

That all looks good. :)

> /*
> * Check whether we have to define FS_IOC_FS[GS]ETXATTR ourselves. These
> * are a copy of the definitions moved to linux/uapi/fs.h in the 4.5 kernel,
> diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c
> index 0d9d7202..2f6a3eb3 100644
> --- a/libxfs/rdwr.c
> +++ b/libxfs/rdwr.c
> @@ -60,9 +60,19 @@ int
> libxfs_device_zero(struct xfs_buftarg *btp, xfs_daddr_t start, uint len)
> {
> xfs_off_t start_offset, end_offset, offset;
> - ssize_t zsize, bytes;
> + ssize_t zsize, bytes, len_bytes;

len_bytes should be size_t, right?

> char *z;
> - int fd;
> + int error, fd;
> +
> + fd = libxfs_device_to_fd(btp->dev);
> + start_offset = LIBXFS_BBTOOFF64(start);
> + end_offset = LIBXFS_BBTOOFF64(start + len) - start_offset;
> +
> + /* try to use special zeroing methods, fall back to writes if needed */
> + len_bytes = LIBXFS_BBTOOFF64(len);
> + error = platform_zero_range(fd, start_offset, len_bytes);

This is a bit ... convoluted, and doesn't end_offset = len_bytes?
i.e.

start_offset = start << BBSHIFT
len_bytes = len << BBSHIFT
end_offset = (start + len) << BBSHIFT - start_offset
= (start << BBSHIFT) + (len << BBSHIFT) - start_offset
= start_offset + len_bytes - start_offset
= len_bytes

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com