Re: [PATCH 05/10] vmscan: Synchrounous lumpy reclaim use lock_page() instead trylock_page()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 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);


Afaik, detailed rule is,

o kswapd can call lock_page() because they never take page lock outside vmscan
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.


>  I did not
> think of an obvious example of when this would happen. Similarly,
> deadlock situations with mmap_sem shouldn't happen unless multiple page
> locks are being taken.
> 
> (prepares to feel foolish)
> 
> What did I miss?





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