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From: mpe@ellerman.id.au (Michael Ellerman)
To: linux-arm-kernel@lists.infradead.org
Subject: linux-next: Tree for Nov 7
Date: Tue, 14 Nov 2017 20:18:04 +1100
Message-ID: <87a7zpw75f.fsf@concordia.ellerman.id.au> (raw)
In-Reply-To: <20171113160637.jhekbdyfpccme3be@dhcp22.suse.cz>

Michal Hocko <mhocko@kernel.org> writes:

> [Sorry for spamming, this one is the last attempt hopefully]
>
> On Mon 13-11-17 16:49:39, Michal Hocko wrote:
>> On Mon 13-11-17 16:16:41, Michal Hocko wrote:
>> > 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. This would
>> > mean that we are exposing a new functionality to the userspace though.
>> > Myabe this would be useful on its own though. 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.
>> 
>> Hmm, the bigger problem would be the backward compatibility actually. We
>> would get silent corruptions which is exactly what the flag is trying
>> fix. mmap flags handling really sucks. So I guess we would have to make
>> the flag internal only :/
>
> OK, so this one should take care of the backward compatibility while
> still not touching the arch code

I'm not sure I understand your worries about backward compatibility?

If we add a new mmap flag which is currently unused then what is the
problem? Are you worried about user code that accidentally passes that
flag already?

> diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> index 203268f9231e..03c518777f83 100644
> --- a/include/uapi/asm-generic/mman-common.h
> +++ b/include/uapi/asm-generic/mman-common.h
> @@ -25,6 +25,8 @@
>  # define MAP_UNINITIALIZED 0x0		/* Don't support this flag */
>  #endif
>  
> +#define MAP_FIXED_SAFE 0x2000000	/* MAP_FIXED which doesn't unmap underlying mapping */
> +

As I said in my other mail I think this should be a modifier to
MAP_FIXED. That way all the existing code that checks for MAP_FIXED (in
the kernel) works exactly as it currently does - like the check Khalid
pointed out.

And I think MAP_NO_CLOBBER would be a better name.

cheers

  parent reply index

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20171107162217.382cd754@canb.auug.org.au>
     [not found] ` <CACPK8Xfd4nqkf=Lk3n6+TNHAAi327r0dkUfGypZ3TpR0LqfS4Q@mail.gmail.com>
     [not found]   ` <20171108142050.7w3yliulxjeco3b7@dhcp22.suse.cz>
     [not found]     ` <20171110123054.5pnefm3mczsfv7bz@dhcp22.suse.cz>
     [not found]       ` <CACPK8Xe5uUKEytkRiszdX511b_cYTD-z3X=ZsMcNJ-NOYnXfuQ@mail.gmail.com>
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 [this message]
2017-11-14  9:29                         ` Michal Hocko
2017-11-14  9:02                   ` Michael Ellerman
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

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