linux-kernel - Re: POWER: Unexpected fault when writing to brk-allocated memory

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Date:   Fri, 10 Nov 2017 04:15:26 +1100
From:   Nicholas Piggin <npiggin@...il.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Florian Weimer <fweimer@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arch@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: POWER: Unexpected fault when writing to brk-allocated memory

On Tue, 7 Nov 2017 17:01:58 +0300
"Kirill A. Shutemov" <kirill@...temov.name> wrote:

> On Tue, Nov 07, 2017 at 07:15:58PM +0530, Aneesh Kumar K.V wrote:
> >   
> > > 
> > > If it is decided to keep these kind of heuristics, can we get just a
> > > small but reasonably precise description of each change to the
> > > interface and ways for using the new functionality, such that would be
> > > suitable for the man page? I couldn't fix powerpc because nothing
> > > matches and even Aneesh and you differ on some details (MAP_FIXED
> > > behaviour).  
> > 
> > 
> > I would consider MAP_FIXED as my mistake. We never discussed this explicitly
> > and I kind of assumed it to behave the same way. ie, we search in lower
> > address space (128TB) if the hint addr is below 128TB.
> > 
> > IIUC we agree on the below.
> > 
> > 1) MAP_FIXED allow the addr to be used, even if hint addr is below 128TB but
> > hint_addr + len is > 128TB.
> > 
> > 2) For everything else we search in < 128TB space if hint addr is below
> > 128TB
> > 
> > 3) We don't switch to large address space if hint_addr + len > 128TB. The
> > decision to switch to large address space is primarily based on hint addr
> > 
> > Is there any other rule we need to outline? Or is any of the above not
> > correct?  
> 
> That's correct.
> 

Thanks guys, I'll send out some powerpc patches to match -- it deviates in
its MAP_FIXED handling (treats it the same as !MAP_FIXED).

So these semantics are what we're going with? Anything that does mmap() is
guaranteed of getting a 47-bit pointer and it can use the top 17 bits for
itself? Is intended to be cross-platform or just x86 and power specific?

Also, this may follow from deduction from 1-3, but for explicit
specification in man page:

4) To get an unspecified allocation with the largest possible address range,
we pass in -1 for mmap hint.

Are we allowing 8 bits bits of unused address in this case, or must the
app not assume anything about number of bits used?

Thanks,
Nick