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

Kirill A. Shutemov
2018-10-12T14:37:28Z

On Fri, Oct 12, 2018 at 03:09:49PM +0100, Anton Ivanov wrote:
> On 10/12/18 2:37 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);
> > +
> > + VM_BUG_ON(!pmd_none(*new_pmd));
> > +
> > + /* Set the new pmd */
> > + set_pmd_at(mm, new_addr, new_pmd, pmd);
>
> UML does not have set_pmd_at at all

Every architecture does. :)

But it may come not from the arch code.

> If I read the code right, MIPS completely ignores the address argument so
> set_pmd_at there may not have the effect which this patch is trying to
> achieve.

Ignoring address is fine. Most architectures do that..
The ideas is to move page table to the new pmd slot. It's nothing to do
with the address passed to set_pmd_at().

--
Kirill A. Shutemov