Commit 2c91bd4a authored by Joel Fernandes (Google)'s avatar Joel Fernandes (Google) Committed by Linus Torvalds
Browse files

mm: speed up mremap by 20x on large regions 

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 speedup is an order of magnitude on x86 (~20x).  On a 1GB mremap,
the mremap completion times drops from 3.4-3.6 milliseconds to 144-160
microseconds.

Before:
Total mremap time for 1GB data: 3521942 nanoseconds.
Total mremap time for 1GB data: 3449229 nanoseconds.
Total mremap time for 1GB data: 3488230 nanoseconds.

After:
Total mremap time for 1GB data: 150279 nanoseconds.
Total mremap time for 1GB data: 144665 nanoseconds.
Total mremap time for 1GB data: 158708 nanoseconds.

If THP is enabled the optimization is mostly skipped except in certain
situations.

[joel@joelfernandes.org: fix 'move_normal_pmd' unused function warning]
  Link: http://lkml.kernel.org/r/20181108224457.GB209347@google.com
Link: http://lkml.kernel.org/r/20181108181201.88826-3-joelaf@google.com


Signed-off-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: default avatarKirill A. Shutemov <kirill@shutemov.name>
Reviewed-by: default avatarWilliam Kucharski <william.kucharski@oracle.com>
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4cf58924

arch/Kconfig

+5 −0
Original line number Diff line number Diff line
@@ -535,6 +535,11 @@ 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

mm/mremap.c

+64 −0
Original line number Diff line number Diff line
@@ -191,6 +191,52 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd, 
		drop_rmap_locks(vma);

}



#ifdef CONFIG_HAVE_MOVE_PMD

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)

{

	spinlock_t *old_ptl, *new_ptl;

	struct mm_struct *mm = vma->vm_mm;

	pmd_t pmd;



	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);

	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);

	flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);

	if (new_ptl != old_ptl)

		spin_unlock(new_ptl);

	spin_unlock(old_ptl);



	return true;

}

#endif



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,
@@ -235,7 +281,25 @@ 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) {

#ifdef 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);

			if (need_rmap_locks)

				drop_rmap_locks(vma);

			if (moved)

				continue;

#endif

		}



		if (pte_alloc(new_vma->vm_mm, new_pmd))

			break;

		next = (new_addr + PMD_SIZE) & PMD_MASK;

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