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

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Date:   Fri, 20 Sep 2019 09:36:12 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Steven Price <Steven.Price@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Matthew Wilcox <willy@...radead.org>,
        Sri Krishna chowdary <schowdary@...dia.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Vineet Gupta <vgupta@...opsys.com>,
        James Hogan <jhogan@...nel.org>,
        Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        linux-snps-arc@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] mm/pgtable/debug: Add test validating architecture
 page table helpers



On 09/18/2019 11:52 PM, Gerald Schaefer wrote:
> On Wed, 18 Sep 2019 18:26:03 +0200
> Christophe Leroy <christophe.leroy@....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.

Like Christophe mentioned earlier on the other thread, we will convert
all __PGTABLE_PXX_FOLDED checks as mm_pxx_folded() but looks like 
ARCH_HAS_[4 and 5]LEVEL_HACK macros will still be around. Will respin
the series with all agreed upon changes first and probably we can then
discuss pending issues from there.

> 
> 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.

Atleast p4d_clear/populate_tests() tests will be taken care.

> 
> 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?
Agreed, it seems better to do the check inside p4d/pud_free() functions.