Re: [PATCH v2 1/2] mm: use aligned address in clear_gigantic_page()

Kefeng Wang
2024-10-28T14:22:20Z

On 2024/10/28 21:46, David Hildenbrand wrote:
> On 28.10.24 14:33, Kefeng Wang wrote:
>>
>>
>> On 2024/10/28 21:14, David Hildenbrand wrote:
>>> On 28.10.24 13:52, Kefeng Wang wrote:
>>>>
>>>>
>>>> On 2024/10/28 18:00, David Hildenbrand wrote:
>>>>> On 26.10.24 07:43, Kefeng Wang wrote:
>>>>>> When clearing gigantic page, it zeros page from the first page to the
>>>>>> last page, if directly passing addr_hint which maybe not the address
>>>>>> of the first page of folio, then some archs could flush the wrong
>>>>>> cache
>>>>>> if it does use the addr_hint as a hint. For non-gigantic page, it
>>>>>> calculates the base address inside, even passed the wrong
>>>>>> addr_hint, it
>>>>>> only has performance impact as the process_huge_page() wants to
>>>>>> process
>>>>>> target page last to keep its cache lines hot), no functional impact.
>>>>>>
>>>>>> Let's pass the real accessed address to folio_zero_user() and use the
>>>>>> aligned address in clear_gigantic_page() to fix it.
>>>>>>
>>>>>> Fixes: 78fefd04c123 ("mm: memory: convert clear_huge_page() to
>>>>>> folio_zero_user()")
>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>>>> ---
>>>>>> v2:
>>>>>> - update changelog to clarify the impact, per Andrew
>>>>>>
>>>>>>     fs/hugetlbfs/inode.c | 2 +-
>>>>>>     mm/memory.c          | 1 +
>>>>>>     2 files changed, 2 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>>>>>> index a4441fb77f7c..a5ea006f403e 100644
>>>>>> --- a/fs/hugetlbfs/inode.c
>>>>>> +++ b/fs/hugetlbfs/inode.c
>>>>>> @@ -825,7 +825,7 @@ static long hugetlbfs_fallocate(struct file
>>>>>> *file,
>>>>>> int mode, loff_t offset,
>>>>>>                 error = PTR_ERR(folio);
>>>>>>                 goto out;
>>>>>>             }
>>>>>> -        folio_zero_user(folio, ALIGN_DOWN(addr, hpage_size));
>>>>>> +        folio_zero_user(folio, addr);
>>>>>>             __folio_mark_uptodate(folio);
>>>>>>             error = hugetlb_add_to_page_cache(folio, mapping, index);
>>>>>>             if (unlikely(error)) {
>>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>>> index 75c2dfd04f72..ef47b7ea5ddd 100644
>>>>>> --- a/mm/memory.c
>>>>>> +++ b/mm/memory.c
>>>>>> @@ -6821,6 +6821,7 @@ static void clear_gigantic_page(struct folio
>>>>>> *folio, unsigned long addr,
>>>>>>         int i;
>>>>>>         might_sleep();
>>>>>> +    addr = ALIGN_DOWN(addr, folio_size(folio));
>>>>>
>>>>> Right, that's what's effectively done in a very bad way in
>>>>> process_huge_page()
>>>>>
>>>>> unsigned long addr = addr_hint &
>>>>>                 ~(((unsigned long)nr_pages << PAGE_SHIFT) - 1);
>>>>>
>>>>>
>>>>> That should all be cleaned up ... process_huge_page() likely shouldn't
>>>>
>>>> Yes, let's fix the bug firstly,
>>>>
>>>>> be even consuming "nr_pages".
>>>>
>>>> No sure about this part, it uses nr_pages as the end and calculate the
>>>> 'base'.
>>>
>>> It should be using folio_nr_pages().
>>
>> But process_huge_page() without an explicit folio argument, I'd like to
>> move the aligned address calculate into the folio_zero_user and
>> copy_user_large_folio(will rename it to folio_copy_user()) in the
>> following cleanup patches, or do it in the fix patches?
>
> First, why does folio_zero_user() call process_huge_page() for *a small
> folio*? Because we like or code to be extra complicated to understand?
> Or am I missing something important?

The folio_zero_user() used for PMD-sized THP and HugeTLB before, and
after anon mTHP supported, it is used for order-2~order-PMD-order THP
and HugeTLB, so it won't process a small folio if I understand correctly.

>
> Second, we should be passing the folio to "process_huge_page" and likely
> rename it to "folio_process_pages()" or sth like that. The function even
> documents "of the specified huge page", but there is none specified. The
> copy case might require a rework.
>
> I think this code needs a serious cleanup ...
>

Yes, I'd like to do more cleanup and rework them, also I find some
performance issue on my arm64 machine[1] which need to be addressed.

[1]