Re: [RFC REBASED 5/5] powerpc/mm/slice: use the dynamic high slice size to limit bitmap operations

[Aneesh Kumar K.V]

On 02/28/2018 12:23 PM, Nicholas Piggin wrote:
> On Tue, 27 Feb 2018 18:11:07 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:
>
> > Nicholas Piggin <npiggin@xxxxxxxxx> writes:
> >
> > > On Tue, 27 Feb 2018 14:31:07 +0530
> > > "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:
> > >   
> > > > Christophe Leroy <christophe.leroy@xxxxxx> writes:
> > > >    
> > > > > The number of high slices a process might use now depends on its
> > > > > address space size, and what allocation address it has requested.
> > > > >
> > > > > This patch uses that limit throughout call chains where possible,
> > > > > rather than use the fixed SLICE_NUM_HIGH for bitmap operations.
> > > > > This saves some cost for processes that don't use very large address
> > > > > spaces.
> > > >
> > > > I haven't really looked at the final code. One of the issue we had was
> > > > with the below scenario.
> > > >
> > > > mmap(addr, len) where addr < 128TB and addr+len > 128TB  We want to make
> > > > sure we build the mask such that we don't find the addr available.
> > >
> > > We should run it through the mmap regression tests. I *think* we moved
> > > all of that logic from the slice code to get_ummapped_area before going
> > > in to slices. I may have missed something though, it would be good to
> > > have more eyes on it.
> > >   
> >
> > mmap(-1,...) failed with the test. Something like below fix it
> >
> > @@ -756,7 +770,7 @@ void slice_set_user_psize(struct mm_struct *mm, unsigned int psize)
> >          mm->context.low_slices_psize = lpsizes;
> >   
> >          hpsizes = mm->context.high_slices_psize;
> > -       high_slices = GET_HIGH_SLICE_INDEX(mm->context.slb_addr_limit);
> > +       high_slices = SLICE_NUM_HIGH;
> >          for (i = 0; i < high_slices; i++) {
> >                  mask_index = i & 0x1;
> >                  index = i >> 1;
> >
> > I guess for everything in the mm_context_t, we should compute it till
> > SLICE_NUM_HIGH. The reason for failure was, even though we recompute the
> > slice mask cached in mm_context on slb_addr_limit, it was still derived
> > from the high_slices_psizes which was computed with lower value.
>
> Okay thanks for catching that Aneesh. I guess that's a slow path so it
> should be okay. Christophe if you're taking care of the series can you
> fold it in? Otherwise I'll do that after yours gets merged.

should we also compute the mm_context_t.slice_mask using SLICE_NUM_HIGH 
and skip the recalc_slice_mask_cache when we change the addr limit?

-aneesh