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

Christian Borntraeger
2018-10-15T07:10:53Z

On 10/12/2018 03:37 AM, Joel Fernandes (Google) wrote:
> 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@kernel.org
> Cc: pantin@google.com
> Cc: hughd@google.com
> Cc: lokeshgidra@google.com
> Cc: dancol@google.com
> Cc: mhocko@kernel.org
> Cc: kirill@shutemov.name
> Cc: akpm@linux-foundation.org
> Signed-off-by: Joel Fernandes (Google) <joel@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);

Adding Martin Schwidefsky.
Is this mapping maybe still in use on other CPUs? If yes, I think for
s390 we need to flush here as well (in other word we might need to introduce
pmd_clear_flush). On s390 you have to use instructions like CRDTE,IPTE or IDTE
to modify page table entries that are still in use. Otherwise you can get a
delayed access exception which is - in contrast to page faults - not recoverable.

> +
> + 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;
>