Re: [PATCH V4] libxfs: use FALLOC_FL_ZERO_RANGE in libxfs_device_zero

Darrick J. Wong
2020-02-25T19:16:52Z

On Tue, Feb 25, 2020 at 10:13:55AM -0800, 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.
>
> V3: make len_bytes a size_t; leave "end_offset" where it is for the loop
> use. It's a bit odd but ... just don't mess with it for now, one patch
> one change.
>
> V4: Use EOPNOTSUPP not EOPNOTSUP (same on linux anyway but meh)
> I ignored(tm) darrick's suggestion to make libxfs_device_zero accept
> a longer length, for now - no callers need anything bigger at this time.

But...

> 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
> +
> /*
> * 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..e2d9d790 100644
> --- a/libxfs/rdwr.c
> +++ b/libxfs/rdwr.c
> @@ -61,8 +61,18 @@ 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;
> + size_t len_bytes;
> char *z;
> - int fd;
> + int error, fd;
> +
> + fd = libxfs_device_to_fd(btp->dev);
> + start_offset = LIBXFS_BBTOOFF64(start);
> +
> + /* try to use special zeroing methods, fall back to writes if needed */
> + len_bytes = LIBXFS_BBTOOFF64(len);

...but if the caller passes in (say) 2^23 daddrs on a 32-bit system,
this conversion will try to stuff 2^32 into a size_t (which is 32-bit),
causing an integer overflow. I grok that no callers currently try this,
but this seems like leaving a logic bomb that could go off on what are
becoming difficult-to-test architectures.

Granted the added overflow checking and whatnot required to convert that
last parameter of libxfs_device_zero to unsigned long long could very
well justify a separate patch for fixing the 64-bitness of the whole
api.

--D

> + error = platform_zero_range(fd, start_offset, len_bytes);
> + if (!error)
> + return 0;
>
> zsize = min(BDSTRAT_SIZE, BBTOB(len));
> if ((z = memalign(libxfs_device_alignment(), zsize)) == NULL) {
> @@ -73,9 +83,6 @@ libxfs_device_zero(struct xfs_buftarg *btp, xfs_daddr_t start, uint len)
> }
> memset(z, 0, zsize);
>
> - fd = libxfs_device_to_fd(btp->dev);
> - start_offset = LIBXFS_BBTOOFF64(start);
> -
> if ((lseek(fd, start_offset, SEEK_SET)) < 0) {
> fprintf(stderr, _("%s: %s seek to offset %llu failed: %s\n"),
> progname, __FUNCTION__,
>
>