linux-ext4 - Re: [PATCH v4 13/27] lib: add errseq_t type and infrastructure for handling it

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Date:   Wed, 10 May 2017 07:29:19 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     NeilBrown <neil@...wn.name>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-cifs@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-mm@...ck.org,
        jfs-discussion@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        cluster-devel@...hat.com, linux-f2fs-devel@...ts.sourceforge.net,
        v9fs-developer@...ts.sourceforge.net, linux-nilfs@...r.kernel.org,
        linux-block@...r.kernel.org
Cc:     dhowells@...hat.com, akpm@...ux-foundation.org, hch@...radead.org,
        ross.zwisler@...ux.intel.com, mawilcox@...rosoft.com,
        jack@...e.com, viro@...iv.linux.org.uk, corbet@....net,
        neilb@...e.de, clm@...com, tytso@....edu, axboe@...nel.dk,
        josef@...icpanda.com, hubcap@...ibond.com, rpeterso@...hat.com,
        bo.li.liu@...cle.com
Subject: Re: [PATCH v4 13/27] lib: add errseq_t type and infrastructure for
 handling it

On Wed, 2017-05-10 at 08:03 +1000, NeilBrown wrote:
> On Tue, May 09 2017, Jeff Layton wrote:
> 
> > An errseq_t is a way of recording errors in one place, and allowing any
> > number of "subscribers" to tell whether an error has been set again
> > since a previous time.
> > 
> > It's implemented as an unsigned 32-bit value that is managed with atomic
> > operations. The low order bits are designated to hold an error code
> > (max size of MAX_ERRNO). The upper bits are used as a counter.
> > 
> > The API works with consumers sampling an errseq_t value at a particular
> > point in time. Later, that value can be used to tell whether new errors
> > have been set since that time.
> > 
> > Note that there is a 1 in 512k risk of collisions here if new errors
> > are being recorded frequently, since we have so few bits to use as a
> > counter. To mitigate this, one bit is used as a flag to tell whether the
> > value has been sampled since a new value was recorded. That allows
> > us to avoid bumping the counter if no one has sampled it since it
> > was last bumped.
> > 
> > Later patches will build on this infrastructure to change how writeback
> > errors are tracked in the kernel.
> > 
> > Signed-off-by: Jeff Layton <jlayton@...hat.com>
> 
> I like that this is a separate lib/*.c - nicely structured too.
> 
> Reviewed-by: NeilBrown <neilb@...e.com>
> 
> 

Thanks, yeah...it occurred to me that this scheme is not really specific
to writeback errors. While I can't think of another use-case for
errseq_t's right offhand, I think this makes for cleaner layering and
should make it easy to use in other ways should they arise.

-- 
Jeff Layton <jlayton@...hat.com>