Messages in this thread Patch in this message | | | Date | Wed, 21 Oct 2015 01:43:53 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH 0/5] MADV_FREE refactoring and fix KSM page |
| |
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
| |