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

Kefeng Wang
2024-10-28T06:36:06Z

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
>