Re: [PATCH 3/4] thp: fix MADV_DONTNEED vs. MADV_FREE race

[Kirill A. Shutemov]

On Mon, Mar 06, 2017 at 10:44:46AM +0900, Minchan Kim wrote:
> Hello, Kirill,
> 
> On Fri, Mar 03, 2017 at 01:26:36PM +0300, Kirill A. Shutemov wrote:
> > On Fri, Mar 03, 2017 at 01:35:11PM +0800, Hillf Danton wrote:
> > > 
> > > On March 02, 2017 11:11 PM Kirill A. Shutemov wrote: 
> > > > 
> > > > Basically the same race as with numa balancing in change_huge_pmd(), but
> > > > a bit simpler to mitigate: we don't need to preserve dirty/young flags
> > > > here due to MADV_FREE functionality.
> > > > 
> > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > > > ---
> > > >  mm/huge_memory.c | 2 --
> > > >  1 file changed, 2 deletions(-)
> > > > 
> > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > index bb2b3646bd78..324217c31ec9 100644
> > > > --- a/mm/huge_memory.c
> > > > +++ b/mm/huge_memory.c
> > > > @@ -1566,8 +1566,6 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > > >  		deactivate_page(page);
> > > > 
> > > >  	if (pmd_young(orig_pmd) || pmd_dirty(orig_pmd)) {
> > > > -		orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
> > > > -			tlb->fullmm);
> > > >  		orig_pmd = pmd_mkold(orig_pmd);
> > > >  		orig_pmd = pmd_mkclean(orig_pmd);
> > > > 
> > > $ grep -n set_pmd_at  linux-4.10/arch/powerpc/mm/pgtable-book3s64.c
> > > 
> > > /*
> > >  * set a new huge pmd. We should not be called for updating
> > >  * an existing pmd entry. That should go via pmd_hugepage_update.
> > >  */
> > > void set_pmd_at(struct mm_struct *mm, unsigned long addr,
> > 
> > +Aneesh.
> > 
> > Urgh... Power is special again.
> > 
> > I think this should work fine.
> > 
> > From 056914fa025992c0a2212aee057c26307ce60238 Mon Sep 17 00:00:00 2001
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Date: Thu, 2 Mar 2017 16:47:45 +0300
> > Subject: [PATCH] thp: fix MADV_DONTNEED vs. MADV_FREE race
> > 
> > Basically the same race as with numa balancing in change_huge_pmd(), but
> > a bit simpler to mitigate: we don't need to preserve dirty/young flags
> > here due to MADV_FREE functionality.
> 
> Could you elaborate a bit more here rather than relying on other
> patch's description?

Okay, updated patch is below.

> And could you say what happens to the userspace if that race
> happens? When I guess from title "MADV_DONTNEED vs MADV_FREE",
> a page cannot be zapped but marked lazyfree or vise versa? Right?

"Vise versa" part should be fine. The case I'm worry about is that
MADV_DONTNEED would skip the pmd and it will not be cleared.
Userspace expects the area of memory to be clean after MADV_DONTNEED, but
it's not. It can lead to userspace misbehaviour.