On Wed, 2010-08-25 at 16:53 -0400, Ted Ts'o wrote:
quoted text
> On Wed, Aug 25, 2010 at 03:35:42PM +0200, Peter Zijlstra wrote:
> > 
> > While I appreciate that it might be somewhat (a lot) harder for a
> > filesystem to provide that guarantee, I'd be deeply worried about your
> > claim that its impossible.
> > 
> > It would render a system without swap very prone to deadlocks. Even with
> > the very tight dirty page accounting we currently have you can fill all
> > your memory with anonymous pages, at which point there's nothing free
> > and you require writeout of dirty pages to succeed.
> 
> For file systems that do delayed allocation, the situation is very
> similar to swapping over NFS.  Sometimes in order to make some free
> memory, you need to spend some free memory... 

Which means you need to be able to compute a bounded amount of that
memory.

quoted text
>  which implies that for
> these file systems, being more aggressive about triggering writeout,
> and being more aggressive about throttling processes which are
> creating too many dirty pages, especially dirty delayed allocaiton
> pages (regardless of whether this is via write(2) or accessing mmapped
> memory), is a really good idea.

That seems unrelated, the VM has a strict dirty limit and controls
writeback when needed. That part works.

quoted text
> A pool of free pages which is reserved for routines that are doing
> page cleaning would probably also be a good idea.  Maybe that's just
> retrying with GFP_ATOMIC if a normal allocation fails, or maybe we
> need our own special pool, or maybe we need to dynamically resize the
> GFP_ATOMIC pool based on how many subsystems might need to use it....

We have a smallish reserve, accessible with PF_MEMALLOC, but its use is
not regulated nor bounded, it just mostly works good enough.


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