Re: [PATCH 00/31] numa/core patches

[Hugh Dickins <hughd@google.com>]

On Wed, 31 Oct 2012, Hugh Dickins wrote:
> On Wed, 31 Oct 2012, Zhouping Liu wrote:
> > On 10/31/2012 03:26 PM, Hugh Dickins wrote:
> > > 
> > > There's quite a few put_page()s in do_huge_pmd_numa_page(), and it
> > > would help if we could focus on the one which is giving the trouble,
> > > but I don't know which that is.  Zhouping, if you can, please would
> > > you do an "objdump -ld vmlinux >bigfile" of your kernel, then extract
> > > from bigfile just the lines from "<do_huge_pmd_numa_page>:" to whatever
> > > is the next function, and post or mail privately just that disassembly.
> > > That should be good to identify which of the put_page()s is involved.
> > 
> > Hugh, I didn't find the next function, as I can't find any words that matched
> > "do_huge_pmd_numa_page".
> > is there any other methods?
> 
> Hmm, do_huge_pmd_numa_page does appear in your stacktrace,
> unless I've made a typo but am blind to it.
> 
> Were you applying objdump to the vmlinux which gave you the
> BUG at mm/memcontrol.c:1134! ?

Thanks for the further info you then sent privately: I have not made any
more effort to reproduce the issue, but your objdump did tell me that the
put_page hitting the problem is the one on line 872 of mm/huge_memory.c,
"Drop the local reference", just before successful return after migration.

I didn't really get the inspiration I'd hoped for out of knowing that,
but it did make wonder whether you're suffering from one of the issues
I already mentioned, and I can now see a way in which it might cause
the mm/memcontrol.c:1134 BUG:-

migrate_page_copy() does TestClearPageActive on the source page:
so given the unsafe way in which do_huge_pmd_numa_page() was proceeding
with a !PageLRU page, it's quite possible that the page was sitting in
a pagevec, and added to the active lru (so added to the lru_size of the
active lru), but our final put_page removes it from lru, active flag has
been cleared, so we subtract it from the lru_size of the inactive lru -
that could indeed make it go negative and trigger the BUG.

Here's a patch fixing and tidying up that and a few other things there.
But I'm not signing it off yet, partly because I've barely tested it
(quite probably I didn't even have any numa pmd migration happening
at all), and partly because just a moment ago I ran across this
instructive comment in __collapse_huge_page_isolate():
	/* cannot use mapcount: can't collapse if there's a gup pin */
	if (page_count(page) != 1) {

Hmm, yes, below I've added the page_mapcount() check I proposed to
do_huge_pmd_numa_page(), but is even that safe enough?  Do we actually
need a page_count() check (for 2?) to guard against get_user_pages()?
I suspect we do, but then do we have enough locking to stabilize such
a check?  Probably, but...

This will take more time, and I doubt get_user_pages() is an issue in
your testing, so please would you try the patch below, to see if it
does fix the BUGs you are seeing?  Thanks a lot.

Not-Yet-Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/huge_memory.c |   24 +++++++++---------------
 1 file changed, 9 insertions(+), 15 deletions(-)

--- 3.7-rc2+schednuma+johannes/mm/huge_memory.c	2012-11-01 04:10:43.812155671 -0700
+++ linux/mm/huge_memory.c	2012-11-01 05:52:19.512153771 -0700
@@ -745,7 +745,7 @@ void do_huge_pmd_numa_page(struct mm_str
 	struct mem_cgroup *memcg = NULL;
 	struct page *new_page = NULL;
 	struct page *page = NULL;
-	int node, lru;
+	int node = -1;
 
 	spin_lock(&mm->page_table_lock);
 	if (unlikely(!pmd_same(*pmd, entry)))
@@ -762,7 +762,8 @@ void do_huge_pmd_numa_page(struct mm_str
 		VM_BUG_ON(!PageCompound(page) || !PageHead(page));
 
 		get_page(page);
-		node = mpol_misplaced(page, vma, haddr);
+		if (page_mapcount(page) == 1)	/* Only do exclusively mapped */
+			node = mpol_misplaced(page, vma, haddr);
 		if (node != -1)
 			goto migrate;
 	}
@@ -801,13 +802,11 @@ migrate:
 	if (!new_page)
 		goto alloc_fail;
 
-	lru = PageLRU(page);
-
-	if (lru && isolate_lru_page(page)) /* does an implicit get_page() */
+	if (isolate_lru_page(page))	/* Does an implicit get_page() */
 		goto alloc_fail;
 
-	if (!trylock_page(new_page))
-		BUG();
+	__set_page_locked(new_page);
+	SetPageSwapBacked(new_page);
 
 	/* anon mapping, we can simply copy page->mapping to the new page: */
 	new_page->mapping = page->mapping;
@@ -820,8 +819,6 @@ migrate:
 	spin_lock(&mm->page_table_lock);
 	if (unlikely(!pmd_same(*pmd, entry))) {
 		spin_unlock(&mm->page_table_lock);
-		if (lru)
-			putback_lru_page(page);
 
 		unlock_page(new_page);
 		ClearPageActive(new_page);	/* Set by migrate_page_copy() */
@@ -829,6 +826,7 @@ migrate:
 		put_page(new_page);		/* Free it */
 
 		unlock_page(page);
+		putback_lru_page(page);
 		put_page(page);			/* Drop the local reference */
 
 		return;
@@ -859,16 +857,12 @@ migrate:
 	mem_cgroup_end_migration(memcg, page, new_page, true);
 	spin_unlock(&mm->page_table_lock);
 
-	put_page(page);			/* Drop the rmap reference */
-
 	task_numa_fault(node, HPAGE_PMD_NR);
 
-	if (lru)
-		put_page(page);		/* drop the LRU isolation reference */
-
 	unlock_page(new_page);
-
 	unlock_page(page);
+	put_page(page);			/* Drop the rmap reference */
+	put_page(page);			/* Drop the LRU isolation reference */
 	put_page(page);			/* Drop the local reference */
 
 	return;

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