[PATCH v2 2/2] mm: speed up mremap by 500x on large regions

Joel Fernandes (Google) joel at joelfernandes.org
Thu Oct 11 18:37:56 PDT 2018


Android needs to mremap large regions of memory during memory management
related operations. The mremap system call can be really slow if THP is
not enabled. The bottleneck is move_page_tables, which is copying each
pte at a time, and can be really slow across a large map. Turning on THP
may not be a viable option, and is not for us. This patch speeds up the
performance for non-THP system by copying at the PMD level when possible.

The speed up is three orders of magnitude. On a 1GB mremap, the mremap
completion times drops from 160-250 millesconds to 380-400 microseconds.

Before:
Total mremap time for 1GB data: 242321014 nanoseconds.
Total mremap time for 1GB data: 196842467 nanoseconds.
Total mremap time for 1GB data: 167051162 nanoseconds.

After:
Total mremap time for 1GB data: 385781 nanoseconds.
Total mremap time for 1GB data: 388959 nanoseconds.
Total mremap time for 1GB data: 402813 nanoseconds.

Incase THP is enabled, the optimization is skipped. I also flush the
tlb every time we do this optimization since I couldn't find a way to
determine if the low-level PTEs are dirty. It is seen that the cost of
doing so is not much compared the improvement, on both x86-64 and arm64.

Cc: minchan at kernel.org
Cc: pantin at google.com
Cc: hughd at google.com
Cc: lokeshgidra at google.com
Cc: dancol at google.com
Cc: mhocko at kernel.org
Cc: kirill at shutemov.name
Cc: akpm at linux-foundation.org
Signed-off-by: Joel Fernandes (Google) <joel at joelfernandes.org>
---
 mm/mremap.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index 9e68a02a52b1..d82c485822ef 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -191,6 +191,54 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
 		drop_rmap_locks(vma);
 }
 
+static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
+		  unsigned long new_addr, unsigned long old_end,
+		  pmd_t *old_pmd, pmd_t *new_pmd, bool *need_flush)
+{
+	spinlock_t *old_ptl, *new_ptl;
+	struct mm_struct *mm = vma->vm_mm;
+
+	if ((old_addr & ~PMD_MASK) || (new_addr & ~PMD_MASK)
+	    || old_end - old_addr < PMD_SIZE)
+		return false;
+
+	/*
+	 * The destination pmd shouldn't be established, free_pgtables()
+	 * should have release it.
+	 */
+	if (WARN_ON(!pmd_none(*new_pmd)))
+		return false;
+
+	/*
+	 * We don't have to worry about the ordering of src and dst
+	 * ptlocks because exclusive mmap_sem prevents deadlock.
+	 */
+	old_ptl = pmd_lock(vma->vm_mm, old_pmd);
+	if (old_ptl) {
+		pmd_t pmd;
+
+		new_ptl = pmd_lockptr(mm, new_pmd);
+		if (new_ptl != old_ptl)
+			spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+
+		/* Clear the pmd */
+		pmd = *old_pmd;
+		pmd_clear(old_pmd);
+
+		VM_BUG_ON(!pmd_none(*new_pmd));
+
+		/* Set the new pmd */
+		set_pmd_at(mm, new_addr, new_pmd, pmd);
+		if (new_ptl != old_ptl)
+			spin_unlock(new_ptl);
+		spin_unlock(old_ptl);
+
+		*need_flush = true;
+		return true;
+	}
+	return false;
+}
+
 unsigned long move_page_tables(struct vm_area_struct *vma,
 		unsigned long old_addr, struct vm_area_struct *new_vma,
 		unsigned long new_addr, unsigned long len,
@@ -239,7 +287,21 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 			split_huge_pmd(vma, old_pmd, old_addr);
 			if (pmd_trans_unstable(old_pmd))
 				continue;
+		} else if (extent == PMD_SIZE) {
+			bool moved;
+
+			/* See comment in move_ptes() */
+			if (need_rmap_locks)
+				take_rmap_locks(vma);
+			moved = move_normal_pmd(vma, old_addr, new_addr,
+					old_end, old_pmd, new_pmd,
+					&need_flush);
+			if (need_rmap_locks)
+				drop_rmap_locks(vma);
+			if (moved)
+				continue;
 		}
+
 		if (pte_alloc(new_vma->vm_mm, new_pmd))
 			break;
 		next = (new_addr + PMD_SIZE) & PMD_MASK;
-- 
2.19.0.605.g01d371f741-goog




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