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

From: Martin Schwidefsky <[email-masked]>
Date: Mon, 15 Oct 2018 10:18:14 +0200

On Mon, 15 Oct 2018 09:10:53 +0200
Christian Borntraeger <[email-masked]> wrote:

> 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: [email-masked]
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Cc: [email-masked]
> > Signed-off-by: Joel Fernandes (Google) <[email-masked]>
> > ---
> >  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.

Just clearing an active pmd would be broken for s390. We need the equivalent
of the ptep_get_and_clear() function for pmds. For s390 this function would
look like this:

static inline pte_t pmdp_get_and_clear(struct mm_struct *mm,
                                       unsigned long addr, pmd_t *pmdp)
{
        return pmdp_xchg_lazy(mm, addr, pmdp, __pmd(_SEGMENT_ENTRY_INVALID));
}

Just like pmdp_huge_get_and_clear() in fact.

> 
> 
> 
> > +
> > +		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;
> > +}
> > +

So the idea is to move the pmd entry to the new location, dragging
the whole pte table to a new location with a different address.
I wonder if that is safe in regard to get_user_pages_fast().

> >  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;
> >   

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.