Re: [patch for-3.17] mm, thp: fix collapsing of hugepages on madvise

From: Kirill A. Shutemov
Date: Sun Oct 05 2014 - 14:41:51 EST


On Sat, Oct 04, 2014 at 07:48:04PM -0700, David Rientjes wrote:
> If an anonymous mapping is not allowed to fault thp memory and then
> madvise(MADV_HUGEPAGE) is used after fault, khugepaged will never
> collapse this memory into thp memory.
>
> This occurs because the madvise(2) handler for thp, hugepage_advise(),
> clears VM_NOHUGEPAGE on the stack and it isn't stored in vma->vm_flags
> until the final action of madvise_behavior(). This causes the
> khugepaged_enter_vma_merge() to be a no-op in hugepage_advise() when the
> vma had previously had VM_NOHUGEPAGE set.
>
> Fix this by passing the correct vma flags to the khugepaged mm slot
> handler. There's no chance khugepaged can run on this vma until after
> madvise_behavior() returns since we hold mm->mmap_sem.
>
> It would be possible to clear VM_NOHUGEPAGE directly from vma->vm_flags
> in hugepage_advise(), but I didn't want to introduce special case
> behavior into madvise_behavior(). I think it's best to just let it
> always set vma->vm_flags itself.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Reported-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>

Look like rather complex fix for a not that complex bug.
What about untested patch below?

From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Date: Sun, 5 Oct 2014 21:22:43 +0300
Subject: [PATCH] thp: fix registering VMA into khugepaged on
madvise(MADV_HUGEPAGE)

hugepage_madvise() tries to register VMA into khugepaged with
khugepaged_enter_vma_merge() on madvise(MADV_HUGEPAGE). Unfortunately
it's effectevely nop, since khugepaged_enter_vma_merge() rely on
vma->vm_flags which has not yet updated by the time of
hugepage_madvise().

Let's move khugepaged_enter_vma_merge() to the end of madvise_behavior().
Now we also have chance to catch VMAs which become good for THP after
vma_merge().

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
mm/huge_memory.c | 8 +++-----
mm/madvise.c | 6 ++++++
2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f8ffd9412ec5..f84d52158a66 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1966,12 +1966,10 @@ int hugepage_madvise(struct vm_area_struct *vma,
*vm_flags &= ~VM_NOHUGEPAGE;
*vm_flags |= VM_HUGEPAGE;
/*
- * If the vma become good for khugepaged to scan,
- * register it here without waiting a page fault that
- * may not happen any time soon.
+ * vma->vm_flags is not yet updated here. madvise_behavior()
+ * will take care to register it in khugepaged once flags
+ * updated.
*/
- if (unlikely(khugepaged_enter_vma_merge(vma)))
- return -ENOMEM;
break;
case MADV_NOHUGEPAGE:
/*
diff --git a/mm/madvise.c b/mm/madvise.c
index 0938b30da4ab..60effd2c5e9c 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -128,6 +128,12 @@ success:
*/
vma->vm_flags = new_flags;

+ /*
+ * If the vma become good for khugepaged to scan, register it here
+ * without waiting a page fault that may not happen any time soon.
+ */
+ if (unlikely(khugepaged_enter_vma_merge(vma)))
+ error = -ENOMEM;
out:
if (error == -ENOMEM)
error = -EAGAIN;
--
Kirill A. Shutemov
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