linux-kernel - Re: [LTP] mmapstress03 weirdness? (fwd)

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Date:	Mon, 21 Sep 2009 18:10:32 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
Cc:	Linux/PPC Development <linuxppc-dev@...abs.org>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	Linux Test Project <Ltp-list@...ts.sourceforge.net>
Subject: Re: [LTP] mmapstress03 weirdness? (fwd)

>         if (mmap((caddr_t)(1UL << (POINTER_SIZE  - 1)) - pagesize,
>                 (size_t)((1UL << (POINTER_SIZE - 1)) - pagesize),
>                 PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_FIXED| 
> MAP_SHARED, 0, 0)
>                 != (caddr_t)-1)

> With 32-bit userland, this boils down to:
>
> | mmap addr 0x7fff0000 size 0x7fff0000
> | mmap returned 0x7fff0000
>
> i.e. mmap() succeeds,

Yes, on a powerpc64 kernel, every 32-bit userspace process has 4GB
available (well, except the lowest few pages).  The process text sits
normally at 1M and the shared libs around 256M.

> but (1) the test expects it to fail, so the test returns
> TFAIL,

That's a bug in the test then.

> but (2) ltp-pan still reports that the tests passed?

Sounds like another bug.

> In addition, sometimes mmapstress03 fails due to SEGV. I created a  
> small test
> program that just does the above mmap(), and depending on the  
> distro and what
> else I print later it crashes with a SEGV, too. Probably this  
> happens because
> the mmap() did succeed, and corrupted some existing mappings,

It probably killed the stack, which sits all the way up near 4G.

> JFYI, with 64-bit userland, this boils down to:
>
> | mmap addr 0x7fffffffffff0000 size 0x7fffffffffff0000
> | mmap returned 0xffffffffffffffff
>
> i.e. mmap() fails as expected, and the test succeeds.

It tries to map space that is reserved for the kernel (c000...)

> Does all of this sound OK?

Seems to me everything works fine, except the tests themselves.


Segher

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