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From: Michael Ellerman <mpe@ellerman.id.au>
To: Michal Hocko <mhocko@kernel.org>
Cc: Joel Stanley <joel@jms.id.au>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux-Next Mailing List <linux-next@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Abdul Haleem <abdhalee@linux.vnet.ibm.com>,
	Ralf Baechle <ralf@linux-mips.org>,
	"James E.J. Bottomley" <jejb@parisc-linux.org>,
	Helge Deller <deller@gmx.de>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>,
	"David S. Miller" <davem@davemloft.net>,
	Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	linux-arm-kernel@lists.infradead.org,
	linuxppc-dev@lists.ozlabs.org, linux-mips@linux-mips.org
Subject: Re: linux-next: Tree for Nov 7
Date: Tue, 14 Nov 2017 20:02:09 +1100
Message-ID: <87efp1w7vy.fsf@concordia.ellerman.id.au> (raw)
In-Reply-To: <20171113151641.yfqrecpcxllpn5mq@dhcp22.suse.cz>

Michal Hocko <mhocko@kernel.org> writes:

> On Mon 13-11-17 13:00:57, Michal Hocko wrote:
> [...]
>> Yes, I have mentioned that in the previous email but the amount of code
>> would be even larger. Basically every arch which reimplements
>> arch_get_unmapped_area would have to special case new MAP_FIXED flag to
>> do vma lookup.
>
> It turned out that this might be much more easier than I thought after
> all. It seems we can really handle that in the common code.

Ah nice. I should have read this before replying to your previous mail.

> This would mean that we are exposing a new functionality to the userspace though.
> Myabe this would be useful on its own though.

Yes I think it would. At least jemalloc seems like it could use it:

  https://github.com/jemalloc/jemalloc/blob/9f455e2786685b443201c33119765c8093461174/src/pages.c#L65

And I have memories of some JIT code I read once which did a loop of
mmap()s or something to try and get allocations below 4GB or some other
limit - but I can't remember now what it was.

> Just a quick draft (not
> even compile tested) whether this makes sense in general. I would be
> worried about unexpected behavior when somebody set other bit without a
> good reason and we might fail with ENOMEM for such a call now.
>
> Elf loader would then use MAP_FIXED_SAFE rather than MAP_FIXED.
> ---
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 3b26cc62dadb..d021c21f9b01 100644
> --- a/arch/alpha/include/uapi/asm/mman.h
> +++ b/arch/alpha/include/uapi/asm/mman.h
> @@ -31,6 +31,9 @@
>  #define MAP_STACK	0x80000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x100000	/* create a huge page mapping */
>  
> +#define MAP_KEEP_MAPPING 0x2000000
> +#define MAP_FIXED_SAFE	MAP_FIXED|MAP_KEEP_MAPPING /* enforce MAP_FIXED without clobbering an existing mapping */


So bike-shedding a bit, but I think "SAFE" is too vague a name.

Perhaps MAP_NO_CLOBBER - which has the single semantic of "do not
clobber any existing mappings".

It would be a flag on its own, so you could pass it with or without
MAP_FIXED, but it would only change the behaviour when MAP_FIXED is
specified also.

cheers

  parent reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-07  5:22 Stephen Rothwell
2017-11-07 22:22 ` Joel Stanley
2017-11-08 14:20   ` Michal Hocko
2017-11-10 12:30     ` Michal Hocko
2017-11-12  1:08       ` Joel Stanley
2017-11-13  9:20         ` Michal Hocko
2017-11-13  9:34           ` Russell King - ARM Linux
2017-11-13  9:42           ` Michal Hocko
2017-11-13 11:34             ` Michael Ellerman
2017-11-13 12:00               ` Michal Hocko
2017-11-13 15:16                 ` Michal Hocko
2017-11-13 15:48                   ` Russell King - ARM Linux
2017-11-13 15:59                     ` Michal Hocko
2017-11-13 15:49                   ` Michal Hocko
2017-11-13 16:06                     ` Michal Hocko
2017-11-13 16:35                       ` Khalid Aziz
2017-11-14  7:07                         ` Michal Hocko
2017-11-14  9:18                       ` Michael Ellerman
2017-11-14  9:29                         ` Michal Hocko
2017-11-14  9:02                   ` Michael Ellerman [this message]
2017-11-14  8:54                 ` Michael Ellerman
2017-11-14  9:04                   ` Michal Hocko
2017-11-14 14:52                     ` Khalid Aziz
2017-11-13 14:11           ` Michal Hocko
2017-11-13 15:09             ` Russell King - ARM Linux
2017-11-13 15:31               ` Michal Hocko
2017-11-14  0:03         ` Andrew Morton
2017-11-14  0:36           ` Stephen Rothwell
  -- strict thread matches above, loose matches on Subject: below --
2019-11-07  8:07 Stephen Rothwell
2018-11-07  3:16 Stephen Rothwell
2013-11-07  7:31 Stephen Rothwell
2012-11-07  5:15 Stephen Rothwell

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