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

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

On 4/8/20 11:51 AM, Yang Shi wrote:
>
>
> 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.

This can't trigger it since MADV_PAGEOUT would isolate the page from lru 
so khugepaged_scan_pmd() can't reach the refcount check at all.

If try_to_unmap() or pageout() fails, the page may stay in swap cache. 
But I didn't think of an easy way to make this happen.

>
>>
>