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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > 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@xxxxxxxxxx>
> > ---
> > 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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