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

[Kirill A. Shutemov]

On Thu, Oct 11, 2018 at 06:37:56PM -0700, 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.

I looked into the code more and noticed move_pte() helper called from
move_ptes(). It changes PTE entry to suite new address.

It is only defined in non-trivial way on Sparc. I don't know much about
Sparc and it's hard for me to say if the optimization will break anything
there.

I think it worth to disable the optimization if __HAVE_ARCH_MOVE_PTE is
defined. Or make architectures state explicitely that the optimization is
safe.

> @@ -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) {

Hm. What guarantees that new_addr is PMD_SIZE-aligned?
It's not obvious to me.

-- 
 Kirill A. Shutemov