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

Martin Schwidefsky
2018-10-15T08:18:14Z

On Mon, 15 Oct 2018 09:10:53 +0200
Christian Borntraeger <borntraeger@de.ibm.com> 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: minchan@kernel.org
> > Cc: pantin@google.com
> > Cc: hughd@google.com
> > Cc: lokeshgidra@google.com
> > Cc: dancol@google.com
> > Cc: mhocko@kernel.org
> > Cc: kirill@shutemov.name
> > Cc: akpm@linux-foundation.org
> > Signed-off-by: Joel Fernandes (Google) <joel@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.

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.