Re: [PATCH 09/13] mm/powerpc: Redefine pXd_huge() with pXd_leaf()

[Christophe Leroy]

Le 14/03/2024 à 13:53, Peter Xu a écrit :
> On Thu, Mar 14, 2024 at 08:45:34AM +0000, Christophe Leroy wrote:
>>
>>
>> Le 13/03/2024 à 22:47, peterx@xxxxxxxxxx a écrit :
>>> From: Peter Xu <peterx@xxxxxxxxxx>
>>>
>>> PowerPC book3s 4K mostly has the same definition on both, except pXd_huge()
>>> constantly returns 0 for hash MMUs.  As Michael Ellerman pointed out [1],
>>> it is safe to check _PAGE_PTE on hash MMUs, as the bit will never be set so
>>> it will keep returning false.
>>>
>>> As a reference, __p[mu]d_mkhuge() will trigger a BUG_ON trying to create
>>> such huge mappings for 4K hash MMUs.  Meanwhile, the major powerpc hugetlb
>>> pgtable walker __find_linux_pte() already used pXd_leaf() to check hugetlb
>>> mappings.
>>>
>>> The goal should be that we will have one API pXd_leaf() to detect all kinds
>>> of huge mappings.  AFAICT we need to use the pXd_leaf() impl (rather than
>>> pXd_huge() ones) to make sure ie. THPs on hash MMU will also return true.
>>
>> All kinds of huge mappings ?
>>
>> pXd_leaf() will detect only leaf mappings (like pXd_huge() ). There are
>> also huge mappings through hugepd. On powerpc 8xx we have 8M huge pages
>> and 512k huge pages. A PGD entry covers 4M so pgd_leaf() won't report
>> those huge pages.
> 
> Ah yes, I should always mention this is in the context of leaf huge pages
> only.  Are the examples you provided all fall into hugepd category?  If so
> I can reword the commit message, as:

On powerpc 8xx, only the 8M huge pages fall into the hugepd case.

The 512k hugepages are at PTE level, they are handled more or less like 
CONT_PTE on ARM. see function set_huge_pte_at() for more context.

You can also look at pte_leaf_size() and pgd_leaf_size().

By the way pgd_leaf_size() looks odd because it is called only when 
pgd_leaf_size() returns true, which never happens for 8M pages.

> 
>          As a reference, __p[mu]d_mkhuge() will trigger a BUG_ON trying to
>          create such huge mappings for 4K hash MMUs.  Meanwhile, the major
>          powerpc hugetlb pgtable walker __find_linux_pte() already used
>          pXd_leaf() to check leaf hugetlb mappings.
> 
>          The goal should be that we will have one API pXd_leaf() to detect
>          all kinds of huge mappings except hugepd.  AFAICT we need to use
>          the pXd_leaf() impl (rather than pXd_huge() ones) to make sure
>          ie. THPs on hash MMU will also return true.
> 
> Does this look good to you?
> 
> Thanks,
>