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:   Tue, 27 Feb 2018 19:11:25 +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 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.

Thanks,
Nick