[Libguestfs] [PATCH 3/3] file: Zero for block devices on old kernels
Eric Blake
eblake at redhat.com
Thu Aug 2 19:39:43 UTC 2018
On 08/02/2018 02:05 PM, Nir Soffer wrote:
> fallocate(FALLOC_FL_ZERO_RANGE) is supportd for block devices with
> modern kernel, but when it is not, we fall back to manual zeroing.
>
> Check if the underlying file is a block device when opening the file,
> and fall back to ioctl(BLKZEROOUT) for aligned zero requests for a
> block device.
>
> +++ b/plugins/file/file.c
> @@ -45,6 +45,7 @@
>
> #if defined(__linux__) && !defined(FALLOC_FL_PUNCH_HOLE)
> #include <linux/falloc.h> /* For FALLOC_FL_*, glibc < 2.18 */
> +#include <linux/fs.h> /* For BLKZEROOUT */
Will this pick up BLKZEROOUT in all cases where it is needed? Or do we
need to relax the !defined(FALLOC_FL_PUNCH_HOLE), and just blindly
include both of these headers for all Linux compilations?
> +static bool
> +is_aligned(struct handle *h, uint64_t n)
> +{
> + return n % h->sector_size == 0;
Since we know (but the compiler doesn't) that sector_size is a power of
2, it is slightly faster to use bitwise math:
return !(n & (h->sector_size - 1))
> +#ifdef BLKSSZGET
> + if (h->is_block_device) {
> + if (ioctl (h->fd, BLKSSZGET, &h->sector_size)) {
> + nbdkit_error ("ioctl(BLKSSZGET): %s: %m", filename);
> + free (h);
> + return NULL;
If the ioctl() fails, would it be better to just fall back...
> + }
> + }
> +#else
> + h->sector_size = 4096; /* Safe guess */
...to the safe guess, instead of giving up entirely? (Might matter on a
system with newer headers that have the macro, but where the kernel does
not support the ioctl).
> @@ -329,6 +361,20 @@ file_zero (void *handle, uint32_t count, uint64_t offset, int may_trim)
> }
> #endif
>
> +#ifdef BLKZEROOUT
> + /* For aligned range and block devices, we can use BLKZEROOUT. */
> + if (h->is_block_device && is_aligned (h, offset) && is_aligned (h, count)) {
Since alignment is a power of 2, you can compress this as:
if (h->is_block_device && is_aligned (h, offset | count)) {
> + uint64_t range[2] = {offset, count};
> +
> + r = ioctl (h->fd, BLKZEROOUT, &range);
> + if (r == 0)
> + return r;
> +
> + nbdkit_error ("zero: %m");
> + return r;
Are we sure that treating ALL errors as fatal is worthwhile, or should
we still attempt to trigger a fall back to writing?
> + }
> +#endif
> +
> /* Trigger a fall back to writing */
> errno = EOPNOTSUPP;
> return r;
>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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