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

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/10/28 14:17, Huang, Ying wrote:
> Kefeng Wang <wangkefeng.wang@huawei.com> writes:
> 
>> 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);
> 
> 'addr' is set with the following statement above,
> 
> 		/* addr is the offset within the file (zero based) */
> 		addr = index * hpage_size;
> 
> So, we just don't need to ALIGN_DOWN() here.  Or do I miss something?

Yes, it is already aligned,
> 
>>   		__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));

but for hugetlb_no_page(),  we do need to align the addr as it use 
vmf->real_address, so I move the alignment into the clear_gigantic_page.

>>   	for (i = 0; i < nr_pages; i++) {
>>   		cond_resched();
>>   		clear_user_highpage(folio_page(folio, i), addr + i * PAGE_SIZE);
> 
> --
> Best Regards,
> Huang, Ying
>