linux-kernel - [PATCH -V5 05/14] hugetlb: Avoid taking i_mmap_mutex in unmap_single

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Date:	Sat,  7 Apr 2012 00:20:51 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	linux-mm@...ck.org, mgorman@...e.de,
	kamezawa.hiroyu@...fujitsu.com, dhillf@...il.com,
	aarcange@...hat.com, mhocko@...e.cz, akpm@...ux-foundation.org,
	hannes@...xchg.org
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: [PATCH -V5 05/14] hugetlb: Avoid taking i_mmap_mutex in unmap_single_vma for hugetlb

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

i_mmap_mutex lock was added in unmap_single_vma by 502717f4e112b18d9c37753a32f675bec9f2838b
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.
( 39dde65c9940c97fcd178a3d2b1c57ed8b7b68aa )

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.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 4b11961..d642b3e 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.rc3.3.g19a6c

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