Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork

[Yang Shi <yang.shi@linux.alibaba.com>]

On 4/8/20 6:10 AM, Kirill A. Shutemov wrote:
> On Mon, Apr 06, 2020 at 01:50:56PM -0700, Yang Shi wrote:
>>
>> On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
>>> The page can be included into collapse as long as it doesn't have extra
>>> pins (from GUP or otherwise).
>>>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>> ---
>>>    mm/khugepaged.c | 25 ++++++++++++++-----------
>>>    1 file changed, 14 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 57ff287caf6b..1e7e6543ebca 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -581,11 +581,18 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>>>    		}
>>>    		/*
>>> -		 * cannot use mapcount: can't collapse if there's a gup pin.
>>> -		 * The page must only be referenced by the scanned process
>>> -		 * and page swap cache.
>>> +		 * Check if the page has any GUP (or other external) pins.
>>> +		 *
>>> +		 * The page table that maps the page has been already unlinked
>>> +		 * from the page table tree and this process cannot get
>>> +		 * additinal pin on the page.
>>> +		 *
>>> +		 * New pins can come later if the page is shared across fork,
>>> +		 * but not for the this process. It is fine. The other process
>>> +		 * cannot write to the page, only trigger CoW.
>>>    		 */
>>> -		if (page_count(page) != 1 + PageSwapCache(page)) {
>>> +		if (total_mapcount(page) + PageSwapCache(page) !=
>>> +				page_count(page)) {
>> This check looks fine for base page, but what if the page is PTE-mapped THP?
>> The following patch made this possible.
>>
>> If it is PTE-mapped THP and the page is in swap cache, the refcount would be
>> 512 + the number of PTE-mapped pages.
>>
>> Shall we do the below change in the following patch?
>>
>> extra_pins = PageSwapCache(page) ? nr_ccompound(page) - 1 : 0;
>> if (total_mapcount(page) + PageSwapCache(page) != page_count(page) -
>> extra_pins) {
>> ...
> Looks like you're right.
>
> It would be nice to have a test case to demonstrate the issue.
>
> Is there any way to trigger moving the page to swap cache? I don't see it
> immediately.

It sounds not easy to trigger since it totally depends on timing, I'm 
wondering we may have to use MADV_PAGEOUT? Something below off the top 
of my head may trigger this?


     CPU       A                                    CPU    B             
             CPU    C
In parent:
MADV_HUGEPAGE
page fault to fill with THP
fork
                                                     In Child:
MADV_NOHUGEPAGE
MADV_DONTNEED (split pmd)
                                                         MADV_PAGEOUT
                                                             -> add_to_swap
                                 khugepaged scan parent and try to 
collapse PTE-mapped

                                                             -> try_to_unmap

When doing MADV_DONTNEED we need make sure head page is unmapped since 
MADV_PAGEOUT would call page_mapcount(page) to skip shared mapping.

>