Re: [PATCH 10/10] mm: per device dirty threshold

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Sat, 2007-04-21 at 22:25 +0200, Miklos Szeredi wrote: 
> > > The other deadlock, in throttle_vm_writeout() is still to be solved.
> > 
> > Let's go back to the original changelog:
> > 
> > Author: marcelo.tosatti <marcelo.tosatti>
> > Date:   Tue Mar 8 17:25:19 2005 +0000
> > 
> >     [PATCH] vm: pageout throttling
> >     
> >     With silly pageout testcases it is possible to place huge amounts of memory
> >     under I/O.  With a large request queue (CFQ uses 8192 requests) it is
> >     possible to place _all_ memory under I/O at the same time.
> >     
> >     This means that all memory is pinned and unreclaimable and the VM gets
> >     upset and goes oom.
> >     
> >     The patch limits the amount of memory which is under pageout writeout to be
> >     a little more than the amount of memory at which balance_dirty_pages()
> >     callers will synchronously throttle.
> >     
> >     This means that heavy pageout activity can starve heavy writeback activity
> >     completely, but heavy writeback activity will not cause starvation of
> >     pageout.  Because we don't want a simple `dd' to be causing excessive
> >     latencies in page reclaim.
> >     
> >     Signed-off-by: Andrew Morton <akpm@osdl.org>
> >     Signed-off-by: Linus Torvalds <torvalds@osdl.org>
> > 
> > (A good one!  I wrote it ;))
> > 
> > 
> > I believe that the combination of dirty-page-tracking and its calls to
> > balance_dirty_pages() mean that we can now never get more than dirty_ratio
> > of memory into the dirty-or-writeback condition.
> > 
> > The vm scanner can convert dirty pages into clean, under-writeback pages,
> > but it cannot increase the total of dirty+writeback.
> 
> What about swapout?  That can increase the number of writeback pages,
> without decreasing the number of dirty pages, no?

Could we not solve that by enabling cap_account_writeback on
swapper_space, and thereby account swap writeback pages. Then the VM
knows it has outstanding IO and need not panic.