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From:   Anshuman Khandual <anshuman.khandual@arm.com>
Subject: Re: [PATCH 1/1] mm/pgtable/debug: Add test validating architecture
 page table helpers
To:     Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
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        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
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Date:   Thu, 5 Sep 2019 14:48:14 +0530
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On 09/05/2019 01:46 AM, Gerald Schaefer wrote:
> On Tue,  3 Sep 2019 13:31:46 +0530
> Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
>> This adds a test module which will validate architecture page table helpers
>> and accessors regarding compliance with generic MM semantics expectations.
>> This will help various architectures in validating changes to the existing
>> page table helpers or addition of new ones.
>>
>> Test page table and memory pages creating it's entries at various level are
>> all allocated from system memory with required alignments. If memory pages
>> with required size and alignment could not be allocated, then all depending
>> individual tests are skipped.
> 
> This looks very useful, thanks. Of course, s390 is quite special and does
> not work nicely with this patch (yet), mostly because of our dynamic page
> table levels/folding. Still need to figure out what can be fixed in the arch

Hmm.

> code and what would need to be changed in the test module. See below for some
> generic comments/questions.

Sure.

> 
> At least one real bug in the s390 code was already revealed by this, which
> is very nice. In pmd/pud_bad(), we also check large pmds/puds for sanity,
> instead of reporting them as bad, which is apparently not how it is expected.

Hmm, so it has already started being useful :)

> 
> [...]
>> +/*
>> + * Basic operations
>> + *
>> + * mkold(entry)			= An old and not a young entry
>> + * mkyoung(entry)		= A young and not an old entry
>> + * mkdirty(entry)		= A dirty and not a clean entry
>> + * mkclean(entry)		= A clean and not a dirty entry
>> + * mkwrite(entry)		= A write and not a write protected entry
>> + * wrprotect(entry)		= A write protected and not a write entry
>> + * pxx_bad(entry)		= A mapped and non-table entry
>> + * pxx_same(entry1, entry2)	= Both entries hold the exact same value
>> + */
>> +#define VADDR_TEST	(PGDIR_SIZE + PUD_SIZE + PMD_SIZE + PAGE_SIZE)
> 
> Why is P4D_SIZE missing in the VADDR_TEST calculation?

This was a random possible virtual address to generate a representative
page table structure for the test. As there is a default (PGDIR_SIZE) for
P4D_SIZE on platforms which really do not have P4D level, it should be okay
to add P4D_SIZE in the above calculation.

> 
> [...]
>> +
>> +#if !defined(__PAGETABLE_PMD_FOLDED) && !defined(__ARCH_HAS_4LEVEL_HACK)
>> +static void pud_clear_tests(pud_t *pudp)
>> +{
>> +	memset(pudp, RANDOM_NZVALUE, sizeof(pud_t));
>> +	pud_clear(pudp);
>> +	WARN_ON(!pud_none(READ_ONCE(*pudp)));
>> +}
> 
> For pgd/p4d/pud_clear(), we only clear if the page table level is present
> and not folded. The memset() here overwrites the table type bits, so
> pud_clear() will not clear anything on s390 and the pud_none() check will
> fail.
> Would it be possible to OR a (larger) random value into the table, so that
> the lower 12 bits would be preserved?

So the suggestion is instead of doing memset() on entry with RANDOM_NZVALUE,
it should OR a large random value preserving lower 12 bits. Hmm, this should
still do the trick for other platforms, they just need non zero value. So on
s390, the lower 12 bits on the page table entry already has valid value while
entering this function which would make sure that pud_clear() really does
clear the entry ?

> 
>> +
>> +static void pud_populate_tests(struct mm_struct *mm, pud_t *pudp, pmd_t *pmdp)
>> +{
>> +	/*
>> +	 * This entry points to next level page table page.
>> +	 * Hence this must not qualify as pud_bad().
>> +	 */
>> +	pmd_clear(pmdp);
>> +	pud_clear(pudp);
>> +	pud_populate(mm, pudp, pmdp);
>> +	WARN_ON(pud_bad(READ_ONCE(*pudp)));
>> +}
> 
> This will populate the pud with a pmd pointer that does not point to the
> beginning of the pmd table, but to the second entry (because of how
> VADDR_TEST is constructed). This will result in failing pud_bad() check
> on s390. Not sure why/how it works on other archs, but would it be possible
> to align pmdp down to the beginning of the pmd table (and similar for the
> other pxd_populate_tests)?

Right, that was a problem. Will fix it by using the saved entries used for
freeing the page table pages at the end, which always point to the beginning
of a page table page.

> 
> [...]
>> +
>> +	p4d_free(mm, saved_p4dp);
>> +	pud_free(mm, saved_pudp);
>> +	pmd_free(mm, saved_pmdp);
>> +	pte_free(mm, (pgtable_t) virt_to_page(saved_ptep));
> 
> pgtable_t is arch-specific, and on s390 it is not a struct page pointer,
> but a pte pointer. So this will go wrong, also on all other archs (if any)
> where pgtable_t is not struct page.
> Would it be possible to use pte_free_kernel() instead, and just pass
> saved_ptep directly?

It makes sense, will change.