Re: [PATCH v3 0/3] vfs: have syncfs() return error when there are writeback errors

[Jeff Layton]

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@xxxxxxxxxx>