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

[Minchan Kim <minchan@kernel.org>]

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@linux.intel.com>
> > > Cc: Minchan Kim <minchan@kernel.org>
> > > ---
> > >  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@linux.intel.com>
> 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?

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?

Thanks.

> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Minchan Kim <minchan@kernel.org>
> ---
>  mm/huge_memory.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bb2b3646bd78..23c1b3d58cf4 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1566,8 +1566,7 @@ 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);
> +		pmdp_invalidate(vma, addr, pmd);
>  		orig_pmd = pmd_mkold(orig_pmd);
>  		orig_pmd = pmd_mkclean(orig_pmd);
>  
> -- 
>  Kirill A. Shutemov
> 
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