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

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Date:   Thu, 1 Mar 2018 19:22:15 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Christophe LEROY <christophe.leroy@....fr>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC REBASED 5/5] powerpc/mm/slice: use the dynamic high slice
 size to limit bitmap operations

On Thu, 1 Mar 2018 08:09:55 +0100
Christophe LEROY <christophe.leroy@....fr> wrote:

> Le 28/02/2018 à 07:53, Nicholas Piggin a écrit :
> > On Tue, 27 Feb 2018 18:11:07 +0530
> > "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:
> >   
> >> Nicholas Piggin <npiggin@...il.com> writes:
> >>  
> >>> On Tue, 27 Feb 2018 14:31:07 +0530
> >>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:
> >>>     
> >>>> Christophe Leroy <christophe.leroy@....fr> 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.
> >   
> 
> I'm not really taking care of your serie, just took it once to see how 
> it fits on the 8xx.
> I prefer if you can handle them. If you need my help for any test on 
> PPC32 don't hesitate to ask.

No problem I can do that. Thanks for rebasing them.

Thanks,
Nick