Messages in this thread | | | Subject | Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork | From | Yang Shi <> | Date | Thu, 9 Apr 2020 17:03:00 -0700 |
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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.
> >> >
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