linux-ext4 - Re: [RFC][PATCH] link.2: AT_ATOMIC_DATA and AT_ATOMIC

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Date:   Fri, 31 May 2019 20:22:06 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Dave Chinner <david@...morbit.com>, Chris Mason <clm@...com>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>,
        Linux Btrfs <linux-btrfs@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [RFC][PATCH] link.2: AT_ATOMIC_DATA and AT_ATOMIC_METADATA

On Fri, May 31, 2019 at 7:41 PM Theodore Ts'o <tytso@....edu> wrote:
>
> On Fri, May 31, 2019 at 06:21:45PM +0300, Amir Goldstein wrote:
> > What do you think of:
> >
> > "AT_ATOMIC_DATA (since Linux 5.x)
> > A filesystem which accepts this flag will guarantee that if the linked file
> > name exists after a system crash, then all of the data written to the file
> > and all of the file's metadata at the time of the linkat(2) call will be
> > visible.
>
> ".... will be visible after the the file system is remounted".  (Never
> hurts to be explicit.)
>
> > The way to achieve this guarantee on old kernels is to call fsync (2)
> > before linking the file, but doing so will also results in flushing of
> > volatile disk caches.
> >
> > A filesystem which accepts this flag does NOT
> > guarantee that any of the file hardlinks will exist after a system crash,
> > nor that the last observed value of st_nlink (see stat (2)) will persist."
> >
>
> This is I think more precise:
>
>     This guarantee can be achieved by calling fsync(2) before linking
>     the file, but there may be more performant ways to provide these
>     semantics.  In particular, note that the use of the AT_ATOMIC_DATA
>     flag does *not* guarantee that the new link created by linkat(2)
>     will be persisted after a crash.

OK. Just to be clear, mentioning hardlinks and st_link is not needed
in your opinion?

>
> We should also document that a file system which does not implement
> this flag MUST return EINVAL if it is passed this flag to linkat(2).
>

OK. I think this part can be documented as possible reason for EINVAL
As in renameat(2) man page:
       EINVAL The filesystem does not support one of the flags in flags.

Thanks,
Amir.