Re: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages

[Kirill A. Shutemov]

On Fri, Mar 27, 2020 at 01:45:55PM -0700, Yang Shi wrote:
> On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
> <kirill@xxxxxxxxxxxxx> wrote:
> >
> > We can collapse PTE-mapped compound pages. We only need to avoid
> > handling them more than once: lock/unlock page only once if it's present
> > in the PMD range multiple times as it handled on compound level. The
> > same goes for LRU isolation and putpack.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++----------
> >  1 file changed, 31 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index b47edfe57f7b..c8c2c463095c 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -515,6 +515,17 @@ void __khugepaged_exit(struct mm_struct *mm)
> >
> >  static void release_pte_page(struct page *page)
> >  {
> > +       /*
> > +        * We need to unlock and put compound page on LRU only once.
> > +        * The rest of the pages have to be locked and not on LRU here.
> > +        */
> > +       VM_BUG_ON_PAGE(!PageCompound(page) &&
> > +                       (!PageLocked(page) && PageLRU(page)), page);
> > +
> > +       if (!PageLocked(page))
> > +               return;
> > +
> > +       page = compound_head(page);
> >         dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
> >         unlock_page(page);
> >         putback_lru_page(page);
> 
> BTW, wouldn't this unlock the whole THP and put it back to LRU?

It is the intention.

> Then we may copy the following PTE mapped pages with page unlocked and
> on LRU. I don't see critical problem, just the pages might be on and off
> LRU by others, i.e. vmscan, compaction, migration, etc. But no one could
> take the page away since try_to_unmap() would fail, but not very
> productive.
> 
> 
> > @@ -537,6 +548,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >         pte_t *_pte;
> >         int none_or_zero = 0, result = 0, referenced = 0;
> >         bool writable = false;
> > +       LIST_HEAD(compound_pagelist);
> >
> >         for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
> >              _pte++, address += PAGE_SIZE) {
> > @@ -561,13 +573,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >                         goto out;
> >                 }
> >
> > -               /* TODO: teach khugepaged to collapse THP mapped with pte */
> > +               VM_BUG_ON_PAGE(!PageAnon(page), page);
> > +
> >                 if (PageCompound(page)) {
> > -                       result = SCAN_PAGE_COMPOUND;
> > -                       goto out;
> > -               }
> > +                       struct page *p;
> > +                       page = compound_head(page);
> >
> > -               VM_BUG_ON_PAGE(!PageAnon(page), page);
> > +                       /*
> > +                        * Check if we have dealt with the compount page
> > +                        * already
> > +                        */
> > +                       list_for_each_entry(p, &compound_pagelist, lru) {
> > +                               if (page ==  p)
> > +                                       break;
> > +                       }
> > +                       if (page ==  p)
> > +                               continue;
> > +               }
> >
> >                 /*
> >                  * We can do it before isolate_lru_page because the
> > @@ -640,6 +662,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >                     page_is_young(page) || PageReferenced(page) ||
> >                     mmu_notifier_test_young(vma->vm_mm, address))
> >                         referenced++;
> > +
> > +               if (PageCompound(page))
> > +                       list_add_tail(&page->lru, &compound_pagelist);
> >         }
> >         if (likely(writable)) {
> >                 if (likely(referenced)) {
> > @@ -1185,11 +1210,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> >                         goto out_unmap;
> >                 }
> >
> > -               /* TODO: teach khugepaged to collapse THP mapped with pte */
> > -               if (PageCompound(page)) {
> > -                       result = SCAN_PAGE_COMPOUND;
> > -                       goto out_unmap;
> > -               }
> > +               page = compound_head(page);
> >
> >                 /*
> >                  * Record which node the original page is from and save this
> > --
> > 2.26.0
> >
> >

-- 
 Kirill A. Shutemov