linux-kernel - Re: per BDI dirty limit (was Re: -mm merge plans for 2.6.24)

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Message-Id: <1191419923.5599.5.camel@lappy>
Date:	Wed, 03 Oct 2007 15:58:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	Fengguang Wu <fengguang.wu@...il.com>, greg@...ah.com
Subject: Re: per BDI dirty limit (was Re: -mm merge plans for 2.6.24)


On Wed, 2007-10-03 at 15:35 +0200, Kay Sievers wrote:
> On Wed, 2007-10-03 at 12:37 +0200, Peter Zijlstra wrote:
> > On Wed, 2007-10-03 at 12:15 +0200, Kay Sievers wrote:
> > > On Tue, 2007-10-02 at 22:05 +1000, Nick Piggin wrote:
> > > > On Tuesday 02 October 2007 21:40, Peter Zijlstra wrote:
> > > > > On Tue, 2007-10-02 at 13:21 +0200, Kay Sievers wrote:
> > > > 
> > > > > > How about adding this information to the tree then, instead of
> > > > > > creating a new top-level hack, just because something that you think
> > > > > > you need doesn't exist.
> > > > >
> > > > > So you suggest adding all the various network filesystems in there
> > > > > (where?), and adding the concept of a BDI, and ensuring all are properly
> > > > > linked together - somehow. Feel free to do so.
> > > > 
> > > > Would something fit better under /sys/fs/? At least filesystems are
> > > > already an existing concept to userspace.
> > > 
> > > Sounds at least less messy than an new top-level directory.
> > > 
> > > But again, if it's "device" releated, like the name suggests, it should
> > > be reachable from the device tree.
> > > Which userspace tool is supposed to set these values, and at what time?
> > > An init-script, something at device discovery/setup? If that is is ever
> > > going to be used in a hotplug setup, you really don't want to go look
> > > for directories with magic device names in another disconnected tree.
> > 
> > Filesystems don't really map to BDIs either. One can have multiple FSs
> > per BDI.
> > 
> > 'Normally' a BDI relates to a block device, but networked (and other
> > non-block device) filesystems have to create a BDI too. So these need to
> > be represented some place as well.
> > 
> > The typical usage would indeed be init scripts. The typical example
> > would be setting the read-ahead window. Currently that cannot be done
> > for NFS mounts.
> 
> What kind of context for a non-block based fs will get the bdi controls
> added? Is there a generic place, or does every non-block based
> filesystem needs to be adapted individually to use it?

Not sure what the other non-block FSs do, but NFS puts it in its
superblock. So that would roughly be per mount.


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