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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 12 Aug 2020 12:04:14 +0200
Message-ID: <CAJfpeguMjU+n-JXE6aUQQGeMpCS4bsy4HQ37NHJ8aD8Aeg2qhA@mail.gmail.com>
Subject: Re: file metadata via fs API
To:     Steven Whitehouse <swhiteho@redhat.com>
Cc:     David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Karel Zak <kzak@redhat.com>,
        Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Nicolas Dichtel <nicolas.dichtel@6wind.com>,
        Christian Brauner <christian@brauner.io>,
        Lennart Poettering <lennart@poettering.net>,
        Linux API <linux-api@vger.kernel.org>,
        Ian Kent <raven@themaw.net>,
        LSM <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Aug 12, 2020 at 11:43 AM Steven Whitehouse <swhiteho@redhat.com> wrote:
>
> Hi,
>
> On 12/08/2020 09:37, Miklos Szeredi wrote:
> [snip]
> >
> > b) The awarded performance boost is not warranted for the use cases it
> > is designed for.

>
> This is a key point. One of the main drivers for this work is the
> efficiency improvement for large numbers of mounts. Ian and Karel have
> already provided performance measurements showing a significant benefit
> compared with what we have today. If you want to propose this
> alternative interface then you need to show that it can sustain similar
> levels of performance, otherwise it doesn't solve the problem. So
> performance numbers here would be helpful.

Definitely.   Will measure performance with the interface which Linus proposed.

I'm not worried, though; the problem with the previous interface was
that it resulted in the complete mount table being re-parsed on each
individual event resulting in quadratic behavior.  This doesn't affect
any interface that can query individual mount/superblock objects.

> Also - I may have missed this earlier in the discussion, what are the
> atomicity guarantees with this proposal? This is the other key point for
> the API, so it would be good to see that clearly stated (i.e. how does
> one use it in combination with the notifications to provide an up to
> date, consistent view of the kernel's mounts)

fsinfo(2) provides version counters on mount and superblock objects to
verify consistency of returned data, since not all data is returned in
a single call.  Same method could be used with the open/read based
interface to verify consistency in case multiple attributes/attribute
groups need to be queried.

Thanks,
Miklos