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

[Kirill A. Shutemov]

On Wed, Oct 24, 2018 at 10:57:33PM +1100, Balbir Singh wrote:
> 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.

pmd_lock() cannot fail, but __pmd_trans_huge_lock() can. We should not
copy the code blindly.

-- 
 Kirill A. Shutemov