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

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Message-ID: <20171107140158.iz4b2lchhrt6eobe@node.shutemov.name>
Date:   Tue, 7 Nov 2017 17:01:58 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     Nicholas Piggin <npiggin@...il.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, 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.

-- 
 Kirill A. Shutemov