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SubjectRe: [PATCH v3 0/3] vfs: have syncfs() return error when there are writeback errors
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On Fri, 2020-02-07 at 12:04 -0500, Jeff Layton wrote:
> You're probably wondering -- Where are v1 and v2 sets?
>
> I did the first couple of versions of this set back in 2018, and then
> got dragged off to work on other things. I'd like to resurrect this set
> though, as I think it's valuable overall, and I have need of it for some
> other work I'm doing.
>
> Currently, syncfs does not return errors when one of the inodes fails to
> be written back. It will return errors based on the legacy AS_EIO and
> AS_ENOSPC flags when syncing out the block device fails, but that's not
> particularly helpful for filesystems that aren't backed by a blockdev.
> It's also possible for a stray sync to lose those errors.
>
> The basic idea is to track writeback errors at the superblock level,
> so that we can quickly and easily check whether something bad happened
> without having to fsync each file individually. syncfs is then changed
> to reliably report writeback errors, and a new ioctl is added to allow
> userland to get at the current errseq_t value w/o having to sync out
> anything.
>
> I do have a xfstest for this. I do not yet have manpage patches, but
> I'm happy to roll some once there is consensus on the interface.
>
> Caveats:
>
> - Having different behavior for an O_PATH descriptor in syncfs is
> a bit odd, but it means that we don't have to grow struct file. Is
> that acceptable from an API standpoint?
>

There are a couple of other options besides requiring an O_PATH fd here:

1) we could just add a new errseq_t field to struct file for this. On my
machine (x86_64) there is 4 bytes of padding at the end of struct file.
An errseq_t would slot in there without changing the slab object size.
YMMV on other arches of course.

2) we could add a new fcntl command value (F_SYNCFS or something?), that
would flip the fd to being suitable for syncfs. If you tried to use the
fd to do a fsync at that point, we could return an error.

Anyone else have other thoughts on how best to do this?

> - This adds a new generic fs ioctl to allow userland to scrape the
> current superblock's errseq_t value. It may be best to present this
> to userland via fsinfo() instead (once that's merged). I'm fine with
> dropping the last patch for now and reworking it for fsinfo if so.
>

To be clear, as I stated in earlier replies, I think we can drop the
ioctl. If we did want something like this, I think we'd want to expose
it via fsinfo() instead, and that could be done after the syncfs changes
went in.

> Jeff Layton (3):
> vfs: track per-sb writeback errors and report them to syncfs
> buffer: record blockdev write errors in super_block that it backs
> vfs: add a new ioctl for fetching the superblock's errseq_t
>
> fs/buffer.c | 2 ++
> fs/ioctl.c | 4 ++++
> fs/open.c | 6 +++---
> fs/sync.c | 9 ++++++++-
> include/linux/errseq.h | 1 +
> include/linux/fs.h | 3 +++
> include/linux/pagemap.h | 5 ++++-
> include/uapi/linux/fs.h | 1 +
> lib/errseq.c | 33 +++++++++++++++++++++++++++++++--
> 9 files changed, 57 insertions(+), 7 deletions(-)
>

--
Jeff Layton <jlayton@kernel.org>

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