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[PATCHv2 15/28] thp: handle file COW faults

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  [PATCHv2 15/28] thp: handle file COW faults "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-02-11 15:30 +0100
    Re: [PATCHv2 15/28] thp: handle file COW faults Dave Hansen <dave.hansen@intel.com> - 2016-02-12 19:40 +0100
      Re: [PATCHv2 15/28] thp: handle file COW faults "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-02-16 11:10 +0100

#1332038 — [PATCHv2 15/28] thp: handle file COW faults

File COW for THP is handled on pte level: just split the pmd.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/memory.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 6c98ed8e3c4a..19eff2164e5b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3334,6 +3334,11 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd)
 	if (fe->vma->vm_ops->pmd_fault)
 		return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd,
 				fe->flags);
+
+	/* COW handled on pte level: split pmd */
+	VM_BUG_ON_VMA(fe->vma->vm_flags & VM_SHARED, fe->vma);
+	split_huge_pmd(fe->vma, fe->pmd, fe->address);
+
 	return VM_FAULT_FALLBACK;
 }
 
-- 
2.7.0

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#1333022

On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote:
> File COW for THP is handled on pte level: just split the pmd.

More changelog.  More comments, please.

We don't want to COW THP's because we'll waste memory?  A COW that we
could handle with 4k, we would have to handle with 2M, and that's
inefficient and high-latency?

Seems like a good idea to me.  It would just be nice to ensure every
reviewer doesn't have to think their way through it.

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#1335238

On Fri, Feb 12, 2016 at 10:36:25AM -0800, Dave Hansen wrote:
> On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote:
> > File COW for THP is handled on pte level: just split the pmd.
> 
> More changelog.  More comments, please.

Okay, I'll add more.

> We don't want to COW THP's because we'll waste memory?  A COW that we
> could handle with 4k, we would have to handle with 2M, and that's
> inefficient and high-latency?

All of above.i

It's not clear how benefitial THP file COW mappings. And it would require
some code to make them work.

I think at some point we can consider teaching khugepaged to collapse such
pages, but allocating huge on fault is probably overkill.

> Seems like a good idea to me.  It would just be nice to ensure every
> reviewer doesn't have to think their way through it.

-- 
 Kirill A. Shutemov

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