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Re: MIPS ASID type conflicts

To: Jean-Christophe PINCE <jcpince@gmail.com>
Subject: Re: MIPS ASID type conflicts
From: Ralf Baechle <ralf@linux-mips.org>
Date: Fri, 2 Nov 2012 10:44:23 +0100
Cc: linux-kernel@vger.kernel.org, linux-mips@linux-mips.org
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On Mon, Apr 23, 2012 at 01:49:26PM +0200, Jean-Christophe PINCE wrote:

> I am analyzing Linux MIPS tasks memory spaces and found out what I
> think is a bug in the ASID management.
> 
> The structure "struct cpuinfo_mips" defined in
> arch/mips/include/asm/cpu-info.h uses a "unsigned int" field for
> asid_cache while the context field defined in
> arch/mips/include/asm/mmu.h is a "unsigned long".
> 
> This is ok with 32bits kernel but leads to 4bytes vs 8bytes fields
> with a 64bits kernel. And when the scheduler checks if the ASID is of
> an older ASID_VERSION, the test will always return that the version
> differs when the context bits above bit31 will be set.
> 
> I imagine this should be a quite rare issue but could likely happen on
> devices running for very long and starting processes very often (or
> running more than 256 processes per cpu). When this condition (bit 32
> or above of asid_cache is set), the effect should be that the TLB will
> be flushed on each context_switch required by the scheduler but there
> shouldn't be any crash.

A full flush of the TLB can be implemented by picking a fresh ASID as
long as there are still fresh ASIDs available.  This happens fairly
frequently; a typical system has burned through the first 256 ASIDs
somewhen during bootup.

There is not much advantage to be gained from having the ASID and generation
counter in a 64-bit variable so I think I'm just going to change
mmu_context_t to:

typedef struct {
        unsigned int asid[NR_CPUS];
        void *vdso;
} mm_context_t;

  Ralf

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