Re: [PATCH V2 2/2] mm/pgtable/debug: Add test validating architecture page table helpers

From: Gerald Schaefer <gerald.schaefer@de.ibm.com>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Mark Rutland <mark.rutland@arm.com>,
	linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
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	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Michal Hocko <mhocko@kernel.org>,
	linux-mm@kvack.org, Paul Mackerras <paulus@samba.org>,
	sparclinux@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	linux-s390@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>,
	x86@kernel.org, Russell King - ARM Linux <linux@armlinux.org.uk>,
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	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH V2 2/2] mm/pgtable/debug: Add test validating architecture page table helpers
Date: Wed, 18 Sep 2019 20:22:43 +0200	[thread overview]
Message-ID: <20190918202243.37e709df@thinkpad> (raw)
In-Reply-To: <64504101-d9dd-f273-02f9-e9a8b178eecc@c-s.fr>

On Wed, 18 Sep 2019 18:26:03 +0200
Christophe Leroy <christophe.leroy@c-s.fr> wrote:

[..] 
> My suggestion was not to completely drop the #ifdef but to do like you 
> did in pgd_clear_tests() for instance, ie to add the following test on 
> top of the function:
> 
> 	if (mm_pud_folded(mm) || is_defined(__ARCH_HAS_5LEVEL_HACK))
> 		return;
> 

Ah, very nice, this would also fix the remaining issues for s390. Since
we have dynamic page table folding, neither __PAGETABLE_PXX_FOLDED nor
__ARCH_HAS_XLEVEL_HACK is defined, but mm_pxx_folded() will work.

mm_alloc() returns with a 3-level page table by default on s390, so we
will run into issues in p4d_clear/populate_tests(), and also at the end
with p4d/pud_free() (double free).

So, adding the mm_pud_folded() check to p4d_clear/populate_tests(),
and also adding mm_p4d/pud_folded() checks at the end before calling
p4d/pud_free(), would make it all work on s390.

BTW, regarding p4d/pud_free(), I'm not sure if we should rather check
the folding inside our s390 functions, similar to how we do it for
p4d/pud_free_tlb(), instead of relying on not being called for folded
p4d/pud. So far, I see no problem with this behavior, all callers of
p4d/pud_free() should be fine because of our folding check within
p4d/pud_present/none(). But that doesn't mean that it is correct not
to check for the folding inside p4d/pud_free(). At least, with this
test module we do now have a caller of p4d/pud_free() on potentially
folded entries, so instead of adding pxx_folded() checks to this
test module, we could add them to our p4d/pud_free() functions.
Any thoughts on this?

Regards,
Gerald

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