linux-kernel - Re: [RFC][PATCH] 9p: add readahead support for loose mode

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Date:	Sun, 16 Sep 2007 22:16:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Eric Van Hensbergen <ericvh@...il.com>,
	v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH] 9p: add readahead support for loose mode

On Sat, 15 Sep 2007 03:41:26 -0700 Andrew Morton
<akpm@...ux-foundation.org> wrote:

> eww, kmap.  Large amounts of them, apparently.
> 
> Be aware that kmap is a) slow and b) deadlockable.  The latter happens when
> multiple tasks want to take more than one kmap simultaneously: they all
> wait for someone else to release one.  Your code here seems especially
> vulnerable to this.
> 
> Nick had a kmap-speedup patchset a while back which addressed a) but I
> don't know if it addressed the deadlock.  But that patch seemed to die.

That would've been me.

What I did to address b) is to pre-allocate each kmap user a second
kmap slot. Of course this isn't water-tight either.

These patches are part of -rt, I could respin against mainline if there
is interest.
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