linux-kernel - Re: [PATCH] writeback: guard against jiffies wraparound on inode->dirtied

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Date:	Tue, 31 Mar 2009 20:50:18 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] writeback: guard against jiffies wraparound on
 inode->dirtied_when checks (try #2)

On Tue, 31 Mar 2009 17:20:31 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 31 Mar 2009 20:03:59 -0400
> Jeff Layton <jlayton@...hat.com> wrote:
> 
> > +		 * It's not sufficient to just do a time_after() check on
> > +		 * dirtied_when. That assumes that dirtied_when will always
> > +		 * change within a period of jiffies that encompasses half the
> > +		 * machine word size (2^31 jiffies on 32-bit arch). That's not
> > +		 * necessarily the case if an inode is being constantly
> > +		 * redirtied. Since dirtied_when can never be in the future,
> > +		 * we can assume that if it appears to be so then it is
> > +		 * actually in the distant past.
> 
> so this really is a 32-bit-only thing.
> 
> I guess that isn't worth optimising for though.
> 

Yeah, it's pretty much impossible to hit this on a 64-bit machine.

> otoh, given that all three comparisons are the same:
> 
> +			time_after(inode->dirtied_when, *older_than_this) &&
> +			time_before_eq(inode->dirtied_when, jiffies))
> 
> (although one is inverted (i think?)), it might end up nicer if this was all done
> in a little helper function?
> 
> That way we only need to comment what's going on at a single site, and
> we could omit the additional test if !CONFIG_64BIT.

Ok, that seems reasonable.

At one point I had a macro similar to time_in_range(), but dropped it
primarily because time_after_but_before_eq() wasn't easy on the eyes.
Thoughts on better names?

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