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

[Minchan Kim <minchan.kim@gmail.com>]

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. :(

-- 
Kind regards,
Minchan Kim


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