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

[Christian Borntraeger]

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 at kernel.org
> Cc: pantin at google.com
> Cc: hughd at google.com
> Cc: lokeshgidra at google.com
> Cc: dancol at google.com
> Cc: mhocko at kernel.org
> Cc: kirill at shutemov.name
> Cc: akpm at linux-foundation.org
> Signed-off-by: Joel Fernandes (Google) <joel at 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;
>