linux-kernel - [RFC PATCH 3/3] hugetlb: Lazy page table copies in fork()

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Message-Id: <20220527225849.284839-4-mike.kravetz@oracle.com>
Date:   Fri, 27 May 2022 15:58:49 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        James Houghton <jthoughton@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: [RFC PATCH 3/3] hugetlb: Lazy page table copies in fork()

Lazy page table copying at fork time was introduced with commit
d992895ba2b2 ("Lazy page table copies in fork()").  At the time,
hugetlb was very new and did not support page faulting.  As a result,
it was excluded.  When full page fault support was added for hugetlb,
the exclusion was not removed.

Simply remove the check that prevents lazy copying of hugetlb page
tables at fork.  Of course, like other mappings this only applies to
shared mappings.

Lazy page table copying at fork will be less advantageous for hugetlb
mappings because:
- There are fewer page table entries with hugetlb
- hugetlb pmds can be shared instead of copied

In any case, completely eliminating the copy at fork time shold speed
things up.

Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 21dadf03f089..f390cc11886d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1245,7 +1245,7 @@ vma_needs_copy(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
 	if (userfaultfd_wp(dst_vma))
 		return true;
 
-	if (src_vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP))
+	if (src_vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
 		return true;
 
 	if (src_vma->anon_vma)
-- 
2.35.3