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SubjectRe: [PATCH 05/10] vmscan: Synchrounous lumpy reclaim use lock_page() instead trylock_page()
On Fri, Sep 10, 2010 at 07:25:43PM +0900, KOSAKI Motohiro wrote:
> > On Thu, Sep 09, 2010 at 01:13:22PM +0900, KOSAKI Motohiro wrote:
> > > > On Thu, 9 Sep 2010 12:04:48 +0900
> > > > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > > >
> > > > > On Mon, 6 Sep 2010 11:47:28 +0100
> > > > > Mel Gorman <mel@csn.ul.ie> wrote:
> > > > >
> > > > > > From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > > > > >
> > > > > > With synchrounous lumpy reclaim, there is no reason to give up to reclaim
> > > > > > pages even if page is locked. This patch uses lock_page() instead of
> > > > > > trylock_page() in this case.
> > > > > >
> > > > > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > > > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > > > >
> > > > > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > > >
> > > > Ah......but can't this change cause dead lock ??
> > >
> > > Yes, this patch is purely crappy. please drop. I guess I was poisoned
> > > by poisonous mushroom of Mario Bros.
> > >
> >
> > Lets be clear on what the exact dead lock conditions are. The ones I had
> > thought about when I felt this patch was ok were;
> >
> > o We are not holding the LRU lock (or any lock, we just called cond_resched())
> > o We do not have another page locked because we cannot lock multiple pages
> > o Kswapd will never be in LUMPY_MODE_SYNC so it is not getting blocked
> > o lock_page() itself is not allocating anything that we could recurse on
>
> True, all.
>
> >
> > One potential dead lock would be if the direct reclaimer held a page
> > lock and ended up here but is that situation even allowed?
>
> example,
>
> __do_fault()
> {
> (snip)
> if (unlikely(!(ret & VM_FAULT_LOCKED)))
> lock_page(vmf.page);
> else
> VM_BUG_ON(!PageLocked(vmf.page));
>
> /*
> * Should we do an early C-O-W break?
> */
> page = vmf.page;
> if (flags & FAULT_FLAG_WRITE) {
> if (!(vma->vm_flags & VM_SHARED)) {
> anon = 1;
> if (unlikely(anon_vma_prepare(vma))) {
> ret = VM_FAULT_OOM;
> goto out;
> }
> page = alloc_page_vma(GFP_HIGHUSER_MOVABLE,
> vma, address);
>

Correct, this is a problem. I already had dropped the patch but thanks for
pointing out a deadlock because I was missing this case. Nothing stops the
page being faulted being sent to shrink_page_list() when alloc_page_vma()
is called. The deadlock might be hard to hit, but it's there.

>
> Afaik, detailed rule is,
>
> o kswapd can call lock_page() because they never take page lock outside vmscan

lock_page_nosync as you point out in your next mail. While it can call
it, kswapd shouldn't because normally it avoids stalls but it would not
deadlock as a result of calling it.

> o if try_lock() is successed, we can call lock_page_nosync() against its page after unlock.
> because the task have gurantee of no lock taken.
> o otherwise, direct reclaimer can't call lock_page(). the task may have a lock already.
>

I think the safer bet is simply to say "direct reclaimers should not
call lock_page() because the fault path could be holding a lock on that
page already".

Thanks.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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