linux-ext4 - Re: fsync() errors is unsafe and risks data loss

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Message-ID: <20180412120122.GE23861@dastard>
Date:   Thu, 12 Apr 2018 22:01:22 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andres Freund <andres@...razel.de>,
        Andreas Dilger <adilger@...ger.ca>,
        20180410184356.GD3563@...nk.org,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "Joshua D. Drake" <jd@...mandprompt.com>
Subject: Re: fsync() errors is unsafe and risks data loss

On Thu, Apr 12, 2018 at 07:09:14AM -0400, Jeff Layton wrote:
> When there is a writeback error, what should be done with the dirty
> page(s)? Right now, we usually just mark them clean and carry on. Is
> that the right thing to do?

There isn't a right thing. Whatever we do will be wrong for someone.

> One possibility would be to invalidate the range that failed to be
> written (or the whole file) and force the pages to be faulted in again
> on the next access. It could be surprising for some applications to not
> see the results of their writes on a subsequent read after such an
> event.

Not to mention a POSIX IO ordering violation. Seeing stale data
after a "successful" write is simply not allowed.

> Maybe that's ok in the face of a writeback error though? IDK.

No matter what we do for async writeback error handling, it will be
slightly different from filesystem to filesystem, not to mention OS
to OS. The is no magic bullet here, so I'm not sure we should worry
too much. There's direct IO for anyone who cares that need to know
about the completion status of every single write IO....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com