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

Anshuman Khandual
2019-09-20T04:18:12Z

On 09/18/2019 11:52 PM, Gerald Schaefer wrote:
> 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.

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.