Re: [patch][rfc] mm: hold page lock over page_mkwrite

[Nick Piggin <npiggin@suse.de>]

On Sun, Mar 01, 2009 at 07:17:44PM +1100, Dave Chinner wrote:
> On Wed, Feb 25, 2009 at 10:36:29AM +0100, Nick Piggin wrote:
> > I need this in fsblock because I am working to ensure filesystem metadata
> > can be correctly allocated and refcounted. This means that page cleaning
> > should not require memory allocation (to be really robust).
> 
> Which, unfortunately, is just a dream for any filesystem that uses
> delayed allocation. i.e. they have to walk the free space trees
> which may need to be read from disk and therefore require memory
> to succeed....

Well it's a dream because probably none of them get it right, but
that doesn't mean its impossible.

You don't need complete memory allocation up-front to be robust,
but having reserves or degraded modes that simply guarantee
forward progress is enough.

For example, if you need to read/write filesystem metadata to find
and allocate free space, then you really only need a page to do all
the IO.

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