Subject: Re: [PATCH 2/4] mm: speed up mremap by 500x on large regions (v2)

From: Balbir Singh <[email-masked]>
Date: Wed, 24 Oct 2018 22:57:33 +1100

On Wed, Oct 24, 2018 at 01:12:56PM +0300, Kirill A. Shutemov wrote:
> On Fri, Oct 12, 2018 at 06:31:58PM -0700, Joel Fernandes (Google) wrote:
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 9e68a02a52b1..2fd163cff406 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) {
> 
> How can it ever be false?
> 
> > +		pmd_t pmd;
> > +
> > +		new_ptl = pmd_lockptr(mm, new_pmd);

Looks like this is largely inspired by move_huge_pmd(), I guess a lot of
the code applies, why not just reuse as much as possible? The same comments
w.r.t mmap_sem helping protect against lock order issues applies as well.

> > +		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);
> > +		if (new_ptl != old_ptl)
> > +			spin_unlock(new_ptl);
> > +		spin_unlock(old_ptl);
> > +
> > +		*need_flush = true;
> > +		return true;
> > +	}
> > +	return false;
> > +}
> > +
> -- 
>  Kirill A. Shutemov
>