linux-ext4 - Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

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Message-ID: <87mvbugz7t.fsf@notabene.neil.brown.name>
Date:   Thu, 06 Apr 2017 10:24:22 +1000
From:   NeilBrown <neilb@...e.com>
To:     Jeff Layton <jlayton@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ext4@...r.kernel.org, akpm@...ux-foundation.org,
        tytso@....edu, jack@...e.cz
Subject: Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

On Wed, Apr 05 2017, Jeff Layton wrote:

>> 
>> O_DIRECT write() can get an EIO from a previous write-back write to the
>> same file.  Maybe non-O_DIRECT writes should too?
>> 
>
> Some already do this for buffered writes.
>
> This is really a philosophical question, IMO...is it correct to return
> an error on a write call, due to writeback failing previously or during
> the write call, quite possibly to a range that the write call does not
> touch? I can see an argument either way for this.

I like the "we already do" argument.

>
> Also, if we do think that returning an error on the write is the right
> thing to do, should that error advance the sequence counter in the
> struct file, such that an fsync afterward gets back 0? My feeling here
> is that fsync should still report an error after a failed write, but
> maybe that's wrong?

My first thought was that one the error has been returned to any
syscall on a given fd, it has been returned.  Once is enough.
My second thought was that maybe your feeling is right.  Having a well
defined error-return point in fsync feels like a nice design.
My third thought was that this would mean either
 - write continues to fail until fsync is called (probably bad), or
 - we need two counters per "struct file", one for fsync, one for write.
   I don't like that much.

So I'm going back to my first thought.

Thanks,
NeilBrown


>
> This is certainly one area where switching to synchronous writes on
> error would make things a little simpler.
> -- 
> Jeff Layton <jlayton@...hat.com>

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