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

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Message-ID: <87y3nh2wt5.fsf@concordia.ellerman.id.au>
Date:   Wed, 08 Nov 2017 15:56:06 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Florian Weimer <fweimer@...hat.com>, linux-arch@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: POWER: Unexpected fault when writing to brk-allocated memory

"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> writes:

>> 
>> 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.

So:
  mmap(0x7ffffffff000, 0x2000, ..., MAP_FIXED ...) = 0x7ffffffff000

> 2) For everything else we search in < 128TB space if hint addr is below 
> 128TB

  mmap((x < 128T), 0x1000, ...) = (y < 128T)
  ...
  mmap(0x7ffffffff000, 0x1000, ...) = 0x7ffffffff000
  mmap(0x800000000000, 0x1000, ...) = 0x800000000000
  ...
  mmap((x >= 128T), 0x1000, ...) = (y >= 128T)

> 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

But does the mmap succeed in that case or not?

ie:  mmap(0x7ffffffff000, 0x2000, ...) = ?

cheers