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

Kefeng Wang
2024-10-28T12:52:39Z

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'.

>
>
> In clear_gigantic_page(), can you please rename the "unsigned long addr"
> parameter to unsigned long "addr_hint" and use an additional "unsigned
> long addr" ?
>

Sure.