linux-kernel - Re: [PATCH v2 1/2] powerpc/set_memory: Avoid spinlock recursion in change_page

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Message-ID: <e07eefc3-6f84-2e95-05f2-8c56eb6a0c1f@csgroup.eu>
Date:   Thu, 23 Dec 2021 15:14:20 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Maxime Bizon <mbizon@...ebox.fr>,
        Russell Currey <ruscur@...sell.cc>
Subject: Re: [PATCH v2 1/2] powerpc/set_memory: Avoid spinlock recursion in
 change_page_attr()



Le 23/12/2021 à 13:09, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@...roup.eu> writes:
>> Commit 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines")
>> included a spin_lock() to change_page_attr() in order to
>> safely perform the three step operations. But then
>> commit 9f7853d7609d ("powerpc/mm: Fix set_memory_*() against
>> concurrent accesses") modify it to use pte_update() and do
>> the operation atomically.
> 
> It's not really atomic, it's just safe against concurrent access.
> 
> We still do a read / modify / write of the pte value.
> 
> Which isn't safe against concurrent calls to change_page_attr() for the
> same address.
> 
> But maybe that's OK? AFAICS other architectures (eg. arm64) have no
> protection against concurrent callers. I think the assumption is higher
> level code is ensuring there's only a single caller at a time.
> 
> On the other hand x86 and s390 do have locking (cpa_lock / cpa_mutex).
> But it seems that's mostly to protect against splitting of page tables,
> which we aren't doing.
> 
> We'd be a bit safer if we used pte_update() "properly", like I did in:
> 
>    https://lore.kernel.org/linuxppc-dev/20210817132552.3375738-1-mpe@ellerman.id.au/
> 
> 

Probably not so simple as that patch, but I get the idea.

See b6cb20fdc273 ("powerpc/book3e: Fix set_memory_x() and set_memory_nx()")

I think we then need to define platform specific helpers to do it, 
similar to ptep_set_wrprotect() and avoid an #ifdefery in change_page_attr()

Christophe