linux-kernel - [PATCH 2/2] mm/thp/pagecache/collapse: Free the pte page table on collapse for thp page cache.

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Date:   Mon, 12 Dec 2016 22:04:28 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     akpm@...ux-foundation.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        mpe@...erman.id.au
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: [PATCH 2/2] mm/thp/pagecache/collapse: Free the pte page table on collapse for thp page cache.

With THP page cache, when trying to build a huge page from regular pte pages,
we just clear the pmd entry. We will take another fault and at that point we
will find the huge page in the radix tree, thereby using the huge page to
complete the page fault

The second fault path will allocate the needed pgtable_t page for archs like
ppc64. So no need to deposit the same in collapse path. Depositing them in
the collapse path resulting in a pgtable_t memory leak also giving errors like
"[ 2362.021762] BUG: non-zero nr_ptes on freeing mm: 3"

Fixes:"mm: THP page cache support for ppc64"

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
---
 mm/khugepaged.c | 21 ++-------------------
 1 file changed, 2 insertions(+), 19 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 7434a63cac94..4e0914849e55 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1242,7 +1242,6 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 	struct vm_area_struct *vma;
 	unsigned long addr;
 	pmd_t *pmd, _pmd;
-	bool deposited = false;
 
 	i_mmap_lock_write(mapping);
 	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
@@ -1267,26 +1266,10 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 			spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
 			/* assume page table is clear */
 			_pmd = pmdp_collapse_flush(vma, addr, pmd);
-			/*
-			 * now deposit the pgtable for arch that need it
-			 * otherwise free it.
-			 */
-			if (arch_needs_pgtable_deposit()) {
-				/*
-				 * The deposit should be visibile only after
-				 * collapse is seen by others.
-				 */
-				smp_wmb();
-				pgtable_trans_huge_deposit(vma->vm_mm, pmd,
-							   pmd_pgtable(_pmd));
-				deposited = true;
-			}
 			spin_unlock(ptl);
 			up_write(&vma->vm_mm->mmap_sem);
-			if (!deposited) {
-				atomic_long_dec(&vma->vm_mm->nr_ptes);
-				pte_free(vma->vm_mm, pmd_pgtable(_pmd));
-			}
+			atomic_long_dec(&vma->vm_mm->nr_ptes);
+			pte_free(vma->vm_mm, pmd_pgtable(_pmd));
 		}
 	}
 	i_mmap_unlock_write(mapping);
-- 
2.10.2