[PATCH -V9 05/15] hugetlb: avoid taking i_mmap_mutex in unmap_single_vma() for hugetlb

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>

i_mmap_mutex lock was added in unmap_single_vma by 502717f4e ("hugetlb:
fix linked list corruption in unmap_hugepage_range()") but we don't use
page->lru in unmap_hugepage_range any more.  Also the lock was taken
higher up in the stack in some code path.  That would result in deadlock.

unmap_mapping_range (i_mmap_mutex)
 -> unmap_mapping_range_tree
    -> unmap_mapping_range_vma
       -> zap_page_range_single
         -> unmap_single_vma
	      -> unmap_hugepage_range (i_mmap_mutex)

For shared pagetable support for huge pages, since pagetable pages are ref
counted we don't need any lock during huge_pmd_unshare.  We do take
i_mmap_mutex in huge_pmd_share while walking the vma_prio_tree in mapping.
(39dde65c9940c97f ("shared page table for hugetlb page")).

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
---
 mm/memory.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 545e18a..f6bc04f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1326,11 +1326,8 @@ static void unmap_single_vma(struct mmu_gather *tlb,
 			 * Since no pte has actually been setup, it is
 			 * safe to do nothing in this case.
 			 */
-			if (vma->vm_file) {
-				mutex_lock(&vma->vm_file->f_mapping->i_mmap_mutex);
+			if (vma->vm_file)
 				__unmap_hugepage_range(tlb, vma, start, end, NULL);
-				mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
-			}
 		} else
 			unmap_page_range(tlb, vma, start, end, details);
 	}
-- 
1.7.10

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