Re: [PATCH 0/5] MADV_FREE refactoring and fix KSM page

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Oct 20, 2015 at 02:36:51PM -0700, Andrew Morton wrote:
> On Tue, 20 Oct 2015 16:21:09 +0900 Minchan Kim <minchan@kernel.org> wrote:
> 
> > 
> > I reviewed THP refcount redesign patch and It seems below patch fixes
> > MADV_FREE problem. It works well for hours.
> > 
> > >From 104a0940b4c0f97e61de9fee0fd602926ff28312 Mon Sep 17 00:00:00 2001
> > From: Minchan Kim <minchan@kernel.org>
> > Date: Tue, 20 Oct 2015 16:00:52 +0900
> > Subject: [PATCH] mm: mark head page dirty in split_huge_page
> > 
> > In thp split in old THP refcount, we mappped all of pages
> > (ie, head + tails) to pte_mkdirty and mark PG_flags to every
> > tail pages.
> > 
> > But with THP refcount redesign, we can lose dirty bit in page table
> > and PG_dirty for head page if we want to free the THP page using
> > migration_entry.
> > 
> > It ends up discarding head page by madvise_free suddenly.
> > This patch fixes it by mark the head page PG_dirty when VM splits
> > the THP page.
> > 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  mm/huge_memory.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index adccfb48ce57..7fbbd42554a1 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3258,6 +3258,7 @@ static void __split_huge_page(struct page *page, struct list_head *list)
> >  	atomic_sub(tail_mapcount, &head->_count);
> >  
> >  	ClearPageCompound(head);
> > +	SetPageDirty(head);
> >  	spin_unlock_irq(&zone->lru_lock);
> >  
> >  	unfreeze_page(page_anon_vma(head), head);
 
Sorry, I've missed the email at first.

> This appears to be a bugfix against Kirill's "thp: reintroduce
> split_huge_page()"?
> 
> Yes, __split_huge_page() is marking the tail pages dirty but forgot
> about the head page
> 
> You say "we can lose dirty bit in page table" but I don't see how the
> above patch fixes that?

I think the problem is in unfreeze_page_vma(), where I missed dirtying
pte.

> Why does __split_huge_page() unconditionally mark the pages dirty, btw?
> Is it because the THP page was known to be dirty?

THP doesn't have backing storage and cannot be swapped out without
splitting, therefore always dirty. (huge zero page is exception, I guess).

> If so, the head page already had PG_dirty, so this patch doesn't do
> anything.

PG_dirty appears on struct page as result of transferring from dirty bit
in page tables. There's no guarantee that it's happened.

> freeze_page(), unfreeze_page() and their callees desperately need some
> description of what they're doing.  Kirill, could you cook somethnig up
> please?

Minchan, could you test patch below instead?

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 86924cc34bac..ea1f3805afa3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3115,7 +3115,7 @@ static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page,
 
                entry = pte_mkold(mk_pte(page, vma->vm_page_prot));
                if (is_write_migration_entry(swp_entry))
-                       entry = maybe_mkwrite(entry, vma);
+                       entry = maybe_mkwrite(pte_mkdirty(entry), vma);
 
                flush_dcache_page(page);
                set_pte_at(vma->vm_mm, address, pte + i, entry);
-- 
 Kirill A. Shutemov

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