linux-kernel - Re: [PATCHv2 3/3] mm: Use updated pmdp_invalidate() inteface to track dirty/accessed bits

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Date:   Fri, 16 Jun 2017 16:21:43 +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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vineet Gupta <vgupta@...opsys.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "David S. Miller" <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 3/3] mm: Use updated pmdp_invalidate() inteface to
 track dirty/accessed bits

On Fri, Jun 16, 2017 at 05:01:30PM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> writes:
> 
> > This patch uses modifed pmdp_invalidate(), that return previous value of pmd,
> > to transfer dirty and accessed bits.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  fs/proc/task_mmu.c |  8 ++++----
> >  mm/huge_memory.c   | 29 ++++++++++++-----------------
> >  2 files changed, 16 insertions(+), 21 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index f0c8b33d99b1..f2fc1ef5bba2 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> 
> .....
> 
> > @@ -1965,7 +1955,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >  	page_ref_add(page, HPAGE_PMD_NR - 1);
> >  	write = pmd_write(*pmd);
> >  	young = pmd_young(*pmd);
> > -	dirty = pmd_dirty(*pmd);
> >  	soft_dirty = pmd_soft_dirty(*pmd);
> >
> >  	pmdp_huge_split_prepare(vma, haddr, pmd);
> > @@ -1995,8 +1984,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >  			if (soft_dirty)
> >  				entry = pte_mksoft_dirty(entry);
> >  		}
> > -		if (dirty)
> > -			SetPageDirty(page + i);
> >  		pte = pte_offset_map(&_pmd, addr);
> >  		BUG_ON(!pte_none(*pte));
> >  		set_pte_at(mm, addr, pte, entry);
> > @@ -2045,7 +2032,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >  	 * and finally we write the non-huge version of the pmd entry with
> >  	 * pmd_populate.
> >  	 */
> > -	pmdp_invalidate(vma, haddr, pmd);
> > +	old = pmdp_invalidate(vma, haddr, pmd);
> > +
> > +	/*
> > +	 * Transfer dirty bit using value returned by pmd_invalidate() to be
> > +	 * sure we don't race with CPU that can set the bit under us.
> > +	 */
> > +	if (pmd_dirty(old))
> > +		SetPageDirty(page);
> > +
> >  	pmd_populate(mm, pmd, pgtable);
> >
> >  	if (freeze) {
> 
> 
> Can we invalidate the pmd early here ? ie, do pmdp_invalidate instead of
> pmdp_huge_split_prepare() ?

I think we can. But it means we would block access to the page for longer
than it's necessary on most architectures. I guess it's not a bit deal.

Maybe as separate patch on top of this patchet? Aneesh, would you take
care of this?

-- 
 Kirill A. Shutemov