On 22.03.21 10:25, ChangLimin wrote:
> For Linux 5.10/5.11, qemu write zeros to a multipath device using
> ioctl(fd, BLKZEROOUT, range) with cache none or directsync return -EBUSY
> permanently.
So as far as I can track back the discussion, Kevin asked on v1 why we’d
set has_write_zeroes to false, i.e. whether the EBUSY might not go away
at some point, and if it did, whether we shouldn’t retry BLKZEROOUT then.
You haven’t explicitly replied to that question (as far as I can see),
so it kind of still stands.
Implicitly, there are two conflicting answers in this patch: On one
hand, the commit message says “permanently”, and this is what you told
Nir as a realistic case where this can occur.
I'm afraid ChangLimin did not answer my question. I'm looking for real
world used case when qemu cannot write zeros to multipath device, when
nobody else is using the device.
I tried to reproduce this on Fedora (kernel 5.10) with qemu-img convert,
once with a multipath device, and once with logical volume on a vg created
on the multipath device, and I could not reproduce this issue.
If I understand the kernel change correctly, this can happen when there is
a mounted file system on top of the multipath device. I don't think we have
a use case when qemu accesses a multipath device when the device is used
by a file system, but maybe I missed something.
So that to me implies
that we actually should not retry BLKZEROOUT, because the EBUSY will
remain, and that condition won’t change while the block device is in use
by qemu.
On the other hand, in the code, you have decided not to reset
has_write_zeroes to false, so the implementation will retry.
EBUSY is usually a temporary error, so retrying makes sense. The question
is if we really can write zeroes manually in this case?
So I don’t quite understand. Should we keep trying BLKZEROOUT or is
there no chance of it working after it has at one point failed with
EBUSY? (Are there other cases besides what’s described in this commit
message where EBUSY might be returned and it is only temporary?)
> Fallback to pwritev instead of exit for -EBUSY error.
>
> The issue was introduced in Linux 5.10:
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=384d87ef2c954fc58e6c5fd8253e4a1984f5fe02
>
> Fixed in Linux 5.12:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=56887cffe946bb0a90c74429fa94d6110a73119d
>
> Signed-off-by: ChangLimin <changlm@chinatelecom.cn>
> ---
> block/file-posix.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 20e14f8e96..d4054ac9cb 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1624,8 +1624,12 @@ static ssize_t
> handle_aiocb_write_zeroes_block(RawPosixAIOData *aiocb)
> } while (errno == EINTR);
>
> ret = translate_err(-errno);
> - if (ret == -ENOTSUP) {
> - s->has_write_zeroes = false;
> + switch (ret) {
> + case -ENOTSUP:
> + s->has_write_zeroes = false; /* fall through */
> + case -EBUSY: /* Linux 5.10/5.11 may return -EBUSY for multipath
> devices */
> + return -ENOTSUP;
> + break;
(Not sure why this break is here.)
Max
> }
> }
> #endif
> --
> 2.27.0
>