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@xxxxxxxxxxxxxxx>
> > ---
> > 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.
--
Kirill A. Shutemov
