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mm: speed up mremap by 20x on large regions

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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: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Reviewed-by: William 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Joel Fernandes (Google) 2019-01-03 15:28:38 -08:00 committed by Linus Torvalds
parent 4cf5892495
commit 2c91bd4a4e
2 changed files with 69 additions and 0 deletions
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5
arch/Kconfig
View File

@ -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

64
mm/mremap.c
View File

@ -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;