linux-kernel - Re: [PATCH v3] mm/hugetlb: fix hugetlb vs. core-mm PT locking

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Message-ID: <3898204d-83c7-4eb9-a4bc-90665b505704@redhat.com>
Date: Thu, 1 Aug 2024 09:35:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Michael Ellerman <mpe@...erman.id.au>, Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 James Houghton <jthoughton@...gle.com>, stable@...r.kernel.org,
 Oscar Salvador <osalvador@...e.de>, Muchun Song <muchun.song@...ux.dev>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v3] mm/hugetlb: fix hugetlb vs. core-mm PT locking

>> pte_alloc_one() ends up calling pte_fragment_alloc(mm, 0). But there we always
>> end up calling pagetable_alloc(, 0).
>>
>> And fragments are supposed to be <= a single page.
>>
>> Now I'm confused what's wrong here ... am I missing something obvious?
>>
>> CCing some powerpc folks. Is this some pte_t oddity?
> 
> It will be because PTRS_PER_PTE is not a compile time constant :(

Oh, sneaky! Thanks a bunch!

> 
>    $ git grep "define PTRS_PER_PTE" arch/powerpc/include/asm/book3s/64
>    arch/powerpc/include/asm/book3s/64/pgtable.h:#define PTRS_PER_PTE        (1 << PTE_INDEX_SIZE)
>    
>    $ git grep "define PTE_INDEX_SIZE" arch/powerpc/include/asm/book3s/64
>    arch/powerpc/include/asm/book3s/64/pgtable.h:#define PTE_INDEX_SIZE  __pte_index_size
>    
>    $ git grep __pte_index_size arch/powerpc/mm/pgtable_64.c
>    arch/powerpc/mm/pgtable_64.c:unsigned long __pte_index_size;
> 
> Which is because the pseries/powernv (book3s64) kernel supports either
> the HPT or Radix MMU at runtime, and they have different page table
> geometry.
> 
> If you change it to use MAX_PTRS_PER_PTE it should work (that's defined
> for all arches).

Yes, that should fly, thanks again!

-- 
Cheers,

David / dhildenb