Re: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Apr 22, 2010 at 11:40:06PM +0900, Minchan Kim wrote:
> On Thu, 2010-04-22 at 23:23 +0900, Minchan Kim wrote:
> > On Thu, 2010-04-22 at 19:51 +0900, KAMEZAWA Hiroyuki wrote:
> > > On Thu, 22 Apr 2010 19:31:06 +0900
> > > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > > 
> > > > On Thu, 22 Apr 2010 19:13:12 +0900
> > > > Minchan Kim <minchan.kim@gmail.com> wrote:
> > > > 
> > > > > On Thu, Apr 22, 2010 at 6:46 PM, KAMEZAWA Hiroyuki
> > > > > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > > > 
> > > > > > Hmm..in my test, the case was.
> > > > > >
> > > > > > Before try_to_unmap:
> > > > > >        mapcount=1, SwapCache, remap_swapcache=1
> > > > > > After remap
> > > > > >        mapcount=0, SwapCache, rc=0.
> > > > > >
> > > > > > So, I think there may be some race in rmap_walk() and vma handling or
> > > > > > anon_vma handling. migration_entry isn't found by rmap_walk.
> > > > > >
> > > > > > Hmm..it seems this kind patch will be required for debug.
> > > > > 
> > > 
> > > Ok, here is my patch for _fix_. But still testing...
> > > Running well at least for 30 minutes, where I can see bug in 10minutes.
> > > But this patch is too naive. please think about something better fix.
> > > 
> > > ==
> > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > 
> > > At adjust_vma(), vma's start address and pgoff is updated under
> > > write lock of mmap_sem. This means the vma's rmap information
> > > update is atoimic only under read lock of mmap_sem.
> > > 
> > > 
> > > Even if it's not atomic, in usual case, try_to_ummap() etc...
> > > just fails to decrease mapcount to be 0. no problem.
> > > 
> > > But at page migration's rmap_walk(), it requires to know all
> > > migration_entry in page tables and recover mapcount.
> > > 
> > > So, this race in vma's address is critical. When rmap_walk meet
> > > the race, rmap_walk will mistakenly get -EFAULT and don't call
> > > rmap_one(). This patch adds a lock for vma's rmap information. 
> > > But, this is _very slow_.
> > > We need something sophisitcated, light-weight update for this..
> > > 
> > > 
> > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > ---
> > >  include/linux/mm_types.h |    1 +
> > >  kernel/fork.c            |    1 +
> > >  mm/mmap.c                |   11 ++++++++++-
> > >  mm/rmap.c                |    3 +++
> > >  4 files changed, 15 insertions(+), 1 deletion(-)
> > > 
> > > Index: linux-2.6.34-rc4-mm1/include/linux/mm_types.h
> > > ===================================================================
> > > --- linux-2.6.34-rc4-mm1.orig/include/linux/mm_types.h
> > > +++ linux-2.6.34-rc4-mm1/include/linux/mm_types.h
> > > @@ -183,6 +183,7 @@ struct vm_area_struct {
> > >  #ifdef CONFIG_NUMA
> > >  	struct mempolicy *vm_policy;	/* NUMA policy for the VMA */
> > >  #endif
> > > +	spinlock_t adjust_lock;
> > >  };
> > >  
> > >  struct core_thread {
> > > Index: linux-2.6.34-rc4-mm1/mm/mmap.c
> > > ===================================================================
> > > --- linux-2.6.34-rc4-mm1.orig/mm/mmap.c
> > > +++ linux-2.6.34-rc4-mm1/mm/mmap.c
> > > @@ -584,13 +584,20 @@ again:			remove_next = 1 + (end > next->
> > >  		if (adjust_next)
> > >  			vma_prio_tree_remove(next, root);
> > >  	}
> > > -
> > > +	/*
> > > +	 * changing all params in atomic. If not, vma_address in rmap.c
> > > + 	 * can see wrong result.
> > > + 	 */
> > > +	spin_lock(&vma->adjust_lock);
> > >  	vma->vm_start = start;
> > >  	vma->vm_end = end;
> > >  	vma->vm_pgoff = pgoff;
> > > +	spin_unlock(&vma->adjust_lock);
> > >  	if (adjust_next) {
> > > +		spin_lock(&next->adjust_lock);
> > >  		next->vm_start += adjust_next << PAGE_SHIFT;
> > >  		next->vm_pgoff += adjust_next;
> > > +		spin_unlock(&next->adjust_lock);
> > >  	}
> > >  
> > >  	if (root) {
> > > @@ -1939,6 +1946,7 @@ static int __split_vma(struct mm_struct 
> > >  	*new = *vma;
> > >  
> > >  	INIT_LIST_HEAD(&new->anon_vma_chain);
> > > +	spin_lock_init(&new->adjust_lock);
> > >  
> > >  	if (new_below)
> > >  		new->vm_end = addr;
> > > @@ -2338,6 +2346,7 @@ struct vm_area_struct *copy_vma(struct v
> > >  			if (IS_ERR(pol))
> > >  				goto out_free_vma;
> > >  			INIT_LIST_HEAD(&new_vma->anon_vma_chain);
> > > +			spin_lock_init(&new_vma->adjust_lock);
> > >  			if (anon_vma_clone(new_vma, vma))
> > >  				goto out_free_mempol;
> > >  			vma_set_policy(new_vma, pol);
> > > Index: linux-2.6.34-rc4-mm1/kernel/fork.c
> > > ===================================================================
> > > --- linux-2.6.34-rc4-mm1.orig/kernel/fork.c
> > > +++ linux-2.6.34-rc4-mm1/kernel/fork.c
> > > @@ -350,6 +350,7 @@ static int dup_mmap(struct mm_struct *mm
> > >  			goto fail_nomem;
> > >  		*tmp = *mpnt;
> > >  		INIT_LIST_HEAD(&tmp->anon_vma_chain);
> > > +		spin_lock_init(&tmp->adjust_lock);
> > >  		pol = mpol_dup(vma_policy(mpnt));
> > >  		retval = PTR_ERR(pol);
> > >  		if (IS_ERR(pol))
> > > Index: linux-2.6.34-rc4-mm1/mm/rmap.c
> > > ===================================================================
> > > --- linux-2.6.34-rc4-mm1.orig/mm/rmap.c
> > > +++ linux-2.6.34-rc4-mm1/mm/rmap.c
> > > @@ -332,11 +332,14 @@ vma_address(struct page *page, struct vm
> > >  	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
> > >  	unsigned long address;
> > >  
> > > +	spin_lock(&vma->adjust_lock);
> > >  	address = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
> > >  	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
> > > +		spin_unlock(&vma->adjust_lock);
> > >  		/* page should be within @vma mapping range */
> > >  		return -EFAULT;
> > >  	}
> > > +	spin_unlock(&vma->adjust_lock);
> > >  	return address;
> > >  }
> > >  
> > 
> > Nice Catch, Kame. :)
> > 
> > For further optimization, we can hold vma->adjust_lock if vma_address
> > returns -EFAULT. But I hope we redesigns it without new locking. 
> > But I don't have good idea, now. :(
> 
> How about this?
> I just merged ideas of Mel and Kame.:)
> 
> It just shows the concept, not formal patch. 
> 
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f90ea92..61ea742 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -578,6 +578,8 @@ again:			remove_next = 1 + (end > next->vm_end);
>  		}
>  	}
>  
> +	if (vma->anon_vma)
> +		spin_lock(&vma->anon_vma->lock);
>  	if (root) {
>  		flush_dcache_mmap_lock(mapping);
>  		vma_prio_tree_remove(vma, root);
> @@ -619,7 +621,8 @@ again:			remove_next = 1 + (end > next->vm_end);
>  
>  	if (mapping)
>  		spin_unlock(&mapping->i_mmap_lock);
> -
> +	if (vma->anon_vma) 
> +		spin_unlock(&vma->anon_vma->lock);
>  	if (remove_next) {
>  		if (file) {
>  			fput(file);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 3a53d9f..8075057 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1359,9 +1359,22 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
>  	spin_lock(&anon_vma->lock);
>  	list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
>  		struct vm_area_struct *vma = avc->vma;
> -		unsigned long address = vma_address(page, vma);
> -		if (address == -EFAULT)
> +		struct anon_vma *tmp_anon_vma = vma->anon_vma;
> +		unsigned long address;
> +		int tmp_vma_lock = 0;
> +		
> +		if (tmp_anon_vma != anon_vma) {
> +			spin_lock(&tmp_anon_vma->lock);	
> +			tmp_vma_lock = 1;
> +		}

heh, I thought of a similar approach at the same time as you but missed
this mail until later. However, with this approach I suspect there is a
possibility that two walkers of the same anon_vma list could livelock if
two locks on the list are held at the same time. Am still thinking of
how it could be resolved without introducing new locking.

> +		address = vma_address(page, vma);
> +		if (address == -EFAULT) {
> +			if (tmp_vma_lock)
> +				spin_unlock(&tmp_anon_vma->lock);
>  			continue;
> +		}
> +		if (tmp_vma_lock)
> +			spin_unlock(&tmp_anon_vma->lock);
>  		ret = rmap_one(page, vma, address, arg);
>  		if (ret != SWAP_AGAIN)
>  			break;
> -- 
> 1.7.0.5
> 
> 
> 
> -- 
> Kind regards,
> Minchan Kim
> 
> 

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

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