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

[Kirill A. Shutemov]

On Fri, Jun 16, 2017 at 12:02:50PM +0900, Minchan Kim wrote:
> Hello,
> 
> On Thu, Jun 15, 2017 at 05:52:24PM +0300, Kirill A. Shutemov wrote:
> > 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@xxxxxxxxxxxxxxx>
> > ---
> >  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
> > @@ -906,13 +906,13 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
> >  static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
> >  		unsigned long addr, pmd_t *pmdp)
> >  {
> > -	pmd_t pmd = *pmdp;
> > +	pmd_t old, pmd = *pmdp;
> >  
> >  	/* See comment in change_huge_pmd() */
> > -	pmdp_invalidate(vma, addr, pmdp);
> > -	if (pmd_dirty(*pmdp))
> > +	old = pmdp_invalidate(vma, addr, pmdp);
> > +	if (pmd_dirty(old))
> >  		pmd = pmd_mkdirty(pmd);
> > -	if (pmd_young(*pmdp))
> > +	if (pmd_young(old))
> >  		pmd = pmd_mkyoung(pmd);
> >  
> >  	pmd = pmd_wrprotect(pmd);
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index a84909cf20d3..0433e73531bf 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1777,17 +1777,7 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> >  	 * pmdp_invalidate() is required to make sure we don't miss
> >  	 * dirty/young flags set by hardware.
> >  	 */
> > -	entry = *pmd;
> > -	pmdp_invalidate(vma, addr, pmd);
> > -
> > -	/*
> > -	 * Recover dirty/young flags.  It relies on pmdp_invalidate to not
> > -	 * corrupt them.
> > -	 */
> > -	if (pmd_dirty(*pmd))
> > -		entry = pmd_mkdirty(entry);
> > -	if (pmd_young(*pmd))
> > -		entry = pmd_mkyoung(entry);
> > +	entry = pmdp_invalidate(vma, addr, pmd);
> >  
> >  	entry = pmd_modify(entry, newprot);
> >  	if (preserve_write)
> > @@ -1927,8 +1917,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >  	struct mm_struct *mm = vma->vm_mm;
> >  	struct page *page;
> >  	pgtable_t pgtable;
> > -	pmd_t _pmd;
> > -	bool young, write, dirty, soft_dirty;
> > +	pmd_t old, _pmd;
> > +	bool young, write, soft_dirty;
> >  	unsigned long addr;
> >  	int i;
> >  
> > @@ -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);
> > +
> 
> When I see this, without this patch, MADV_FREE has been broken because
> it can lose dirty bit by early checking. Right?
> If so, isn't it a candidate for -stable?

Actually, I don't see how MADV_FREE supposed to work: vmscan splits THP on
reclaim and split_huge_page() would set unconditionally, so MADV_FREE
seems no effect on THP.

Or have I missed anything?

-- 
 Kirill A. Shutemov