Re: [PATCH 3/9] VFS: Introduce a mount context

[Miklos Szeredi <mszeredi@redhat.com>]

On Wed, May 10, 2017 at 3:20 PM, Jeff Layton <jlayton@redhat.com> wrote:
> On Wed, 2017-05-10 at 09:05 +0100, David Howells wrote:
>> Miklos Szeredi <mszeredi@redhat.com> wrote:
>>
>> > Possible rule of thumb: use it only at the place where the error
>> > originates and not where errors are just passed on.  This would result
>> > in at most one report per syscall, normally.
>> >
>
> That might be hard to enforce in practice once you get into some
> complicated layering. What if we have device_mapper setting this along
> with filesystems too? We need clear rules here.

If the error originates in the devicemapper, then why would the
filesystem set it?

There's always a root cause of an error and that should be where the
detailed error is set.

Am I missing something?

>
>> > And the static string thing that David implemented is also a very good
>> > idea, IMO.
>>
>> There is an issue with it: it's fine as long as you keep a ref on the module
>> that generated it or clear all strings as part of module removal (which the
>> mount context in this patchset does).  With the NFS mount context I did, I
>> have to keep a ref on the NFS protocol module as well as the NFS filesystem
>> module.
>>
>> I'm tempted to make it conditionally copy the string using kvasprintf_const()
>> - which would also permit format substitution.
>>
>
> On balance, I think this is a reasonable way to pass back detailed
> errors. Up until now, we've mostly relied on just printk'ing them. Now
> though, a lot of larger machines are running containerized setups. Good
> luck scraping dmesg for _your_ error in that situation. There may be
> tons of mounts failing all over the place.
>
> That said, I have some concerns here:
>
> What's the lifetime of these strings? Do they just hang around forever
> until the process goes away or they're replaced? If this becomes common,
> then you could easily end up with an extra string allocation per task in
> some cases. That could add up.

That's why I liked the static string thing.  It's just one assignment
and no worries about freeing.  Not sure what to do about modules,
though.  Can we somehow move the cost of checking the validity to the
place where the error is retrieved?

>
> One idea might be to always kfree it on syscall entry, and that might
> mitigate the problem assuming that not everything is erroring out. Then
> you could always do some trivial syscall to clear it manually.
>
> There's also the problem of how these should be formatted. Is English ok
> everywhere? Do we need a facility to allow translating these things?

Messages in dmesg are in English too.  If necessary userspace will do
the translation.  I don't think the kernel would need to worry about
that.

Thanks,
Miklos


> --
> Jeff Layton <jlayton@redhat.com>