Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask

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

Le 05/09/2022 à 10:37, David Hildenbrand a écrit :
> On 03.09.22 09:07, Christophe Leroy wrote:
>> +Resending with valid powerpc list address
>>
>> Le 02/09/2022 à 20:52, David Hildenbrand a écrit :
>>>>>> Adding Christophe on Cc:
>>>>>>
>>>>>> Christophe do you know if is_hugepd is true for all hugetlb 
>>>>>> entries, not
>>>>>> just hugepd?
>>
>> is_hugepd() is true if and only if the directory entry points to a huge
>> page directory and not to the normal lower level directory.
>>
>> As far as I understand if the directory entry is not pointing to any
>> lower directory but is a huge page entry, pXd_leaf() is true.
>>
>>
>>>>>>
>>>>>> On systems without hugepd entries, I guess ptdump skips all 
>>>>>> hugetlb entries.
>>>>>> Sigh!
>>
>> As far as I can see, ptdump_pXd_entry() handles the pXd_leaf() case.
>>
>>>>>
>>>>> IIUC, the idea of ptdump_walk_pgd() is to dump page tables even 
>>>>> outside
>>>>> VMAs (for debugging purposes?).
>>>>>
>>>>> I cannot convince myself that that's a good idea when only holding the
>>>>> mmap lock in read mode, because we can just see page tables getting
>>>>> freed concurrently e.g., during concurrent munmap() ... while holding
>>>>> the mmap lock in read we may only walk inside VMA boundaries.
>>>>>
>>>>> That then raises the questions if we're only calling this on 
>>>>> special MMs
>>>>> (e.g., init_mm) whereby we cannot really see concurrent munmap() and
>>>>> where we shouldn't have hugetlb mappings or hugepd entries.
>>
>> At least on powerpc, PTDUMP handles only init_mm.
>>
>> Hugepage are used at least on powerpc 8xx for linear memory mapping, see
>>
>> commit 34536d780683 ("powerpc/8xx: Add a function to early map kernel
>> via huge pages")
>> commit cf209951fa7f ("powerpc/8xx: Map linear memory with huge pages")
>>
>> hugepds may also be used in the future to use huge pages for vmap and
>> vmalloc, see commit a6a8f7c4aa7e ("powerpc/8xx: add support for huge
>> pages on VMAP and VMALLOC")
>>
>> As far as I know, ppc64 also use huge pages for VMAP and VMALLOC, see
>>
>> commit d909f9109c30 ("powerpc/64s/radix: Enable HAVE_ARCH_HUGE_VMAP")
>> commit 8abddd968a30 ("powerpc/64s/radix: Enable huge vmalloc mappings")
> 
> There is a difference between an ordinary huge mapping (e.g., as used 
> for THP) and a a hugetlb mapping.
> 
> Our current understanding is that hugepd only applies to hugetlb. 
> Wouldn't vmap/vmalloc user ordinary huge pmd entries instead of hugepd?
> 

'hugepd' stands for huge page directory. It is independant of whether a 
huge page is used for hugetlb or for anything else, it represents the 
way pages are described in the page tables.

I don't know what you mean by _ordinary_ huge pmd entry.

Let's take the exemple of powerpc 8xx which is the one I know best. This 
is a powerpc32, so it has two levels : PGD and PTE. PGD has 1024 entries 
and each entry covers a 4Mbytes area. Normal PTE has 1024 entries and 
each entry is a 4k page. When you use 8Mbytes pages, you don't use PTEs 
as it would be a waste of memory. You use a huge page directory that has 
a single entry, and you have two PGD entries pointing to the huge page 
directory.

Some time ago, hupgepd was also used for 512kbytes pages and 16kbytes 
pages:
- there was huge page directories with 8x 512kbytes pages,
- there was huge page directories with 256x 16kbytes pages,

And the PGD/PMD entry points to a huge page directory (HUGEPD) instead 
of pointing to a page table directory (PTE).

Since commit b250c8c08c79 ("powerpc/8xx: Manage 512k huge pages as 
standard pages."), the 8xx doesn't use anymore hugepd for 512k huge 
page, but other platforms like powerpc book3e extensively use huge page 
directories.

I hope this clarifies the subject, otherwise I'm happy to provide 
further details.

Christophe