Re: [pve-devel] [PATCH zfsonlinux] patches: fix for zvol sync/flush regression

[Stoiko Ivanov <s.ivanov@proxmox.com>]

Thanks for analysis and preparing the patch!

LGTM - gave it a spin on a VM (with nested guest for testing) on kernel
6.8 - works as expected.

Tested-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-by: Stoiko Ivanov <s.ivanov@proxmox.com>

On Tue, 11 Mar 2025 09:44:03 +0100
Fabian Grünbichler <f.gruenbichler@proxmox.com> wrote:

> this broke with 2.2.7, and can potentially cause data loss or
> inconsistency. the patch basically reverts to pre-2.2.7 behaviour,
> verified via a fio benchmark.
> 
> reported on our forum: https://forum.proxmox.com/threads/163066
> 
> upstream PR: https://github.com/openzfs/zfs/pull/17131
> 
> Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
> ---
> module only change, needs kernel bump to be active
> 
>  ...vols-correctly-detect-flush-requests.patch | 58 +++++++++++++++++++
>  debian/patches/series                         |  1 +
>  2 files changed, 59 insertions(+)
>  create mode 100644 debian/patches/0012-linux-zvols-correctly-detect-flush-requests.patch
> 
> diff --git a/debian/patches/0012-linux-zvols-correctly-detect-flush-requests.patch b/debian/patches/0012-linux-zvols-correctly-detect-flush-requests.patch
> new file mode 100644
> index 0000000..609934a
> --- /dev/null
> +++ b/debian/patches/0012-linux-zvols-correctly-detect-flush-requests.patch
> @@ -0,0 +1,58 @@
> +From c9ae14e064f961709557fdcbdb40e692a97225ec Mon Sep 17 00:00:00 2001
> +From: =?UTF-8?q?Fabian=20Gr=C3=BCnbichler?= <f.gruenbichler@proxmox.com>
> +Date: Mon, 10 Mar 2025 15:51:09 +0100
> +Subject: [PATCH] linux: zvols: correctly detect flush requests
> +MIME-Version: 1.0
> +Content-Type: text/plain; charset=UTF-8
> +Content-Transfer-Encoding: 8bit
> +
> +since 4.10, bio->bi_opf needs to be checked to determine all kinds of
> +flush requests. this was the case prior to the commit referenced below,
> +but the order of ifdefs was not the usual one (newest up top), which
> +might have caused this to slip through.
> +
> +this fixes a regression when using zvols as Qemu block devices, but
> +might have broken other use cases as well. the symptoms are that all
> +sync writes from within a VM configured to use such a virtual block
> +devices are ignored and treated as async writes by the host ZFS layer.
> +
> +this can be verified using fio in sync mode inside the VM, for example
> +with
> +
> + fio \
> + --filename=/dev/sda --ioengine=libaio --loops=1 --size=10G \
> + --time_based --runtime=60 --group_reporting --stonewall --name=cc1 \
> + --description="CC1" --rw=write --bs=4k --direct=1 --iodepth=1 \
> + --numjobs=1 --sync=1
> +
> +which shows an IOPS number way above what the physical device underneath
> +supports, with "zpool iostat -r 1" on the hypervisor side showing no
> +sync IO occuring during the benchmark.
> +
> +with the regression fixed, both fio inside the VM and the IO stats on
> +the host show the expected numbers.
> +
> +Fixes: 846b5985192467acabf5484ae610b4b37b7f8162
> +"config: remove HAVE_REQ_OP_* and HAVE_REQ_*"
> +
> +Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
> +---
> + include/os/linux/kernel/linux/blkdev_compat.h | 2 +-
> + 1 file changed, 1 insertion(+), 1 deletion(-)
> +
> +diff --git a/include/os/linux/kernel/linux/blkdev_compat.h b/include/os/linux/kernel/linux/blkdev_compat.h
> +index d96708c60..084869c34 100644
> +--- a/include/os/linux/kernel/linux/blkdev_compat.h
> ++++ b/include/os/linux/kernel/linux/blkdev_compat.h
> +@@ -383,7 +383,7 @@ bio_set_flush(struct bio *bio)
> + static inline boolean_t
> + bio_is_flush(struct bio *bio)
> + {
> +-	return (bio_op(bio) == REQ_OP_FLUSH);
> ++	return (bio_op(bio) == REQ_OP_FLUSH) || (op_is_flush(bio->bi_opf));
> + }
> + 
> + /*
> +-- 
> +2.39.5
> +
> diff --git a/debian/patches/series b/debian/patches/series
> index 229027f..938488c 100644
> --- a/debian/patches/series
> +++ b/debian/patches/series
> @@ -9,3 +9,4 @@
>  0009-arc-stat-summary-guard-access-to-freshly-introduced-.patch
>  0010-Fix-nfs_truncate_shares-without-etc-exports.d.patch
>  0011-zpool-status-tighten-bounds-for-noalloc-stat-availab.patch
> +0012-linux-zvols-correctly-detect-flush-requests.patch



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