[folded-merged] khugepaged-enable-collapse-pmd-for-pte-mapped-thp-fix.patch removed from -mm tree

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



The patch titled
     Subject: khugepaged: add comments for retract_page_tables()
has been removed from the -mm tree.  Its filename was
     khugepaged-enable-collapse-pmd-for-pte-mapped-thp-fix.patch

This patch was dropped because it was folded into khugepaged-enable-collapse-pmd-for-pte-mapped-thp.patch

------------------------------------------------------
From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Subject: khugepaged: add comments for retract_page_tables()

Oleg Nesterov pointed that logic behind checks in retract_page_tables()
are not obvious.

Add comments to clarify the reasoning for the checks and why they are
safe.

Link: http://lkml.kernel.org/r/20190816145443.6ard3iilytc6jlgv@box
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Song Liu <songliubraving@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/khugepaged.c |   28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

--- a/mm/khugepaged.c~khugepaged-enable-collapse-pmd-for-pte-mapped-thp-fix
+++ a/mm/khugepaged.c
@@ -1421,7 +1421,22 @@ static void retract_page_tables(struct a
 
 	i_mmap_lock_write(mapping);
 	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
-		/* probably overkill */
+		/*
+		 * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
+		 * got written to. These VMAs are likely not worth investing
+		 * down_write(mmap_sem) as PMD-mapping is likely to be split
+		 * later.
+		 *
+		 * Not that vma->anon_vma check is racy: it can be set up after
+		 * the check but before we took mmap_sem by the fault path.
+		 * But page lock would prevent establishing any new ptes of the
+		 * page, so we are safe.
+		 *
+		 * An alternative would be drop the check, but check that page
+		 * table is clear before calling pmdp_collapse_flush() under
+		 * ptl. It has higher chance to recover THP for the VMA, but
+		 * has higher cost too.
+		 */
 		if (vma->anon_vma)
 			continue;
 		addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
@@ -1434,9 +1449,10 @@ static void retract_page_tables(struct a
 			continue;
 		/*
 		 * We need exclusive mmap_sem to retract page table.
-		 * If trylock fails we would end up with pte-mapped THP after
-		 * re-fault. Not ideal, but it's more important to not disturb
-		 * the system too much.
+		 *
+		 * We use trylock due to lock inversion: we need to acquire
+		 * mmap_sem while holding page lock. Fault path does it in
+		 * reverse order. Trylock is a way to avoid deadlock.
 		 */
 		if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
 			spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
@@ -1446,8 +1462,10 @@ static void retract_page_tables(struct a
 			up_write(&vma->vm_mm->mmap_sem);
 			mm_dec_nr_ptes(vma->vm_mm);
 			pte_free(vma->vm_mm, pmd_pgtable(_pmd));
-		} else
+		} else {
+			/* Try again later */
 			khugepaged_add_pte_mapped_thp(vma->vm_mm, addr);
+		}
 	}
 	i_mmap_unlock_write(mapping);
 }
_

Patches currently in -mm which might be from kirill.shutemov@xxxxxxxxxxxxxxx are

khugepaged-enable-collapse-pmd-for-pte-mapped-thp.patch




[Index of Archives]     [Kernel Archive]     [IETF Annouce]     [DCCP]     [Netdev]     [Networking]     [Security]     [Bugtraq]     [Yosemite]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux SCSI]

  Powered by Linux