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

Kefeng Wang
2024-10-28T13:33:54Z

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?