Re: [PATCH] mm: Speed up mremap on large regions

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Oct 09, 2018 at 02:38:59PM -0700, Andrew Morton wrote:
> On Tue,  9 Oct 2018 13:14:00 -0700 "Joel Fernandes (Google)" <joel@joelfernandes.org> 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.
> 
> Looks tasty.

Thanks :)

> > --- 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);
> >  }
> >  
> > +bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
> 
> I'll park this for now, shall plan to add a `static' in there then
> merge it up after 4.20-rc1.

Thanks, I will also add static to the function in my own tree just for the
future in case I'm doing another revision.

- Joel