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@xxxxxxxxxx
Cc: pantin@xxxxxxxxxx
Cc: hughd@xxxxxxxxxx
Cc: lokeshgidra@xxxxxxxxxx
Cc: dancol@xxxxxxxxxx
Cc: mhocko@xxxxxxxxxx
Cc: kirill@xxxxxxxxxxxxx
Cc: akpm@xxxxxxxxxxxxxxxxxxxx
Cc: kernel-team@xxxxxxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
arch/Kconfig | 5 ++++
mm/mremap.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 70 insertions(+)
diff --git a/arch/Kconfig b/arch/Kconfig
index 6801123932a5..9724fe39884f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -518,6 +518,11 @@ config HAVE_IRQ_TIME_ACCOUNTING
Archs need to ensure they use a high enough resolution clock to
support irq time accounting and then call enable_sched_clock_irqtime().
+config HAVE_MOVE_PMD
+ bool
+ help
+ Archs that select this are able to move page tables at the PMD level.
+
config HAVE_ARCH_TRANSPARENT_HUGEPAGE
bool
diff --git a/mm/mremap.c b/mm/mremap.c
index 9e68a02a52b1..2fd163cff406 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 +287,24 @@ 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 && IS_ENABLED(CONFIG_HAVE_MOVE_PMD)) {
+ /*
+ * If the extent is PMD-sized, try to speed the move by
+ * moving at the PMD level if possible.
+ */
+ bool moved;
+
+ 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
