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

[KAMEZAWA Hiroyuki]

On Thu, 22 Apr 2010 19:31:06 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Thu, 22 Apr 2010 19:13:12 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> 
> > On Thu, Apr 22, 2010 at 6:46 PM, KAMEZAWA Hiroyuki
> > <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> 
> > > Hmm..in my test, the case was.
> > >
> > > Before try_to_unmap:
> > > Â Â Â Âmapcount=1, SwapCache, remap_swapcache=1
> > > After remap
> > > Â Â Â Âmapcount=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@xxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxx>
---
 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;
 }
 




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