linux-kernel - Re: [PATCH 3/4] thp: fix MADV_DONTNEED vs. MADV

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Date:   Tue, 7 Mar 2017 16:52:58 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 3/4] thp: fix MADV_DONTNEED vs. MADV_FREE race

On Mon, Mar 06, 2017 at 08:19:03AM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> writes:
> 
> > 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@...ux.intel.com>
> > Cc: Minchan Kim <minchan@...nel.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);
> >  
> 
> Instead can we do a new interface that does something like
> 
> pmdp_huge_update(tlb->mm, addr, pmd, new_pmd);
> 
> We do have a variant already in ptep_set_access_flags. What we need is
> something that can be used to update THP pmd, without converting it to
> pmd_none and one which doens't loose reference and change bit ?

Sounds like a good idea. Would you volunteer to implement it?
I don't have time for this right now.

-- 
 Kirill A. Shutemov