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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 18/03/2024 à 17:15, Jason Gunthorpe a écrit :
> On Thu, Mar 14, 2024 at 01:11:59PM +0000, Christophe Leroy wrote:
>>
>>
>> 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@redhat.com a écrit :
>>>>> From: Peter Xu <peterx@redhat.com>
>>>>>
>>>>> 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().
> 
> IMHO leaf should return false if the thing is pointing to a next level
> page table, even if that next level is fully populated with contiguous
> pages.
> 
> This seems more aligned with the contig page direction that hugepd
> should be moved over to..

Should hugepd be moved to the contig page direction, really ?

Would it be acceptable that a 8M hugepage requires 2048 contig entries 
in 2 page tables, when the hugepd allows a single entry ? Would it be 
acceptable performancewise ?

> 
>> 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.
> 
> Like this, you should reach the actual final leaf that the HW will
> load and leaf_size() should say it is greater size than the current
> table level. Other levels should return 0.
> 
> If necessary the core MM code should deal with this by iterating over
> adjacent tables.
> 
> Jason