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:   Wed, 28 Feb 2018 16:53:31 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        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 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.

Thanks,
Nick