Re: [PATCH] thp+memcg-numa: fix BUG at include/linux/mm.h:370!

[Andrea Arcangeli <aarcange@redhat.com>]

On Mon, Mar 14, 2011 at 08:58:23PM +0100, Johannes Weiner wrote:
> We don't care about the vma.  It's all about assigning the physical
> page to the memcg that mm->owner belongs to.
> 
> It would be the first callsite not holding the mmap_sem, but that is
> only because all existing sites are fault handlers that don't drop the
> lock for other reasons.

I was afraid it'd be the first callsite this is why I wasn't excited
to push it in 2.6.38, but Linus's right and we should micro-optimize
it for 2.6.39.

> I am not aware of anything that would rely on the lock in there, or
> would not deserve to break if it did.

Thanks for double checking.

What about this? (only problem is the thp-vmstat patch in -mm then
reject, maybe I should rediff it against -mm instead, as you wish)

===
Subject: thp: optimize memcg charge in khugepaged

From: Andrea Arcangeli <aarcange@redhat.com>

We don't need to hold the mmmap_sem through mem_cgroup_newpage_charge(), the
mmap_sem is only hold for keeping the vma stable and we don't need the vma
stable anymore after we return from alloc_hugepage_vma().

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0a619e0..c61d9ad 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1762,12 +1762,9 @@ static void collapse_huge_page(struct mm_struct *mm,
 
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 #ifndef CONFIG_NUMA
+	up_read(&mm->mmap_sem);
 	VM_BUG_ON(!*hpage);
 	new_page = *hpage;
-	if (unlikely(mem_cgroup_newpage_charge(new_page, mm, GFP_KERNEL))) {
-		up_read(&mm->mmap_sem);
-		return;
-	}
 #else
 	VM_BUG_ON(*hpage);
 	/*
@@ -1782,20 +1779,20 @@ static void collapse_huge_page(struct mm_struct *mm,
 	 */
 	new_page = alloc_hugepage_vma(khugepaged_defrag(), vma, address,
 				      node, __GFP_OTHER_NODE);
+	/* after allocating the hugepage upgrade to mmap_sem write mode */
+	up_read(&mm->mmap_sem);
 	if (unlikely(!new_page)) {
-		up_read(&mm->mmap_sem);
 		*hpage = ERR_PTR(-ENOMEM);
 		return;
 	}
+#endif
+
 	if (unlikely(mem_cgroup_newpage_charge(new_page, mm, GFP_KERNEL))) {
-		up_read(&mm->mmap_sem);
+#ifdef CONFIG_NUMA
 		put_page(new_page);
+#endif
 		return;
 	}
-#endif
-
-	/* after allocating the hugepage upgrade to mmap_sem write mode */
-	up_read(&mm->mmap_sem);
 
 	/*
 	 * Prevent all access to pagetables with the exception of

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