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 <[email protected]>
> > Signed-off-by: Linus Torvalds <[email protected]>
> >
> > (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.
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