linux-kernel - Re: [PATCH 00/14] VFS: Filesystem information [ver #18]

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Date:   Mon, 9 Mar 2020 17:18:21 -0700
From:   Andres Freund <andres@...razel.de>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     David Howells <dhowells@...hat.com>, torvalds@...ux-foundation.org,
        viro@...iv.linux.org.uk, Theodore Ts'o <tytso@....edu>,
        Stefan Metzmacher <metze@...ba.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        linux-nfs@...r.kernel.org, linux-api@...r.kernel.org,
        raven@...maw.net, mszeredi@...hat.com, christian@...uner.io,
        jannh@...gle.com, darrick.wong@...cle.com, kzak@...hat.com,
        linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/14] VFS: Filesystem information [ver #18]

Hi,

On 2020-03-09 18:49:31 -0400, Jeff Layton wrote:
> On Mon, 2020-03-09 at 12:22 -0700, Andres Freund wrote:
> > On 2020-03-09 13:50:59 -0400, Jeff Layton wrote:
> > > I sent a patch a few weeks ago to make syncfs() return errors when there
> > > have been writeback errors on the superblock. It's not merged yet, but
> > > once we have something like that in place, we could expose info from the
> > > errseq_t to userland using this interface.
> >
> > I'm still a bit worried about the details of errseq_t being exposed to
> > userland. Partially because it seems to restrict further evolution of
> > errseq_t, and partially because it will likely up with userland trying
> > to understand it (it's e.g. just too attractive to report a count of
> > errors etc).
>
> Trying to interpret the counter field won't really tell you anything.
> The counter is not incremented unless someone has queried the value
> since it was last checked. A single increment could represent a single
> writeback error or 10000 identical ones.

Oh, right.  A zero errseq would still indicate something, but that's
probably fine.


> > Is there a reason to not instead report a 64bit counter instead of the
> > cookie? In contrast to the struct file case we'd only have the space
> > overhead once per superblock, rather than once per #files * #fd. And it
> > seems that the maintenance of that counter could be done without
> > widespread changes, e.g. instead/in addition to your change:

> What problem would moving to a 64-bit counter solve? I get the concern
> about people trying to get a counter out of the cookie field, but giving
> people an explicit 64-bit counter seems even more open to
> misinterpretation.

Well, you could get an actual error count out of it? I was thinking that
that value would get incremented every time mapping_set_error() is
called, which should make it a meaningful count?

Greetings,

Andres Freund