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@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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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