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

[Minchan Kim <minchan@kernel.org>]

On Fri, Jun 16, 2017 at 04:19:08PM +0300, Kirill A. Shutemov wrote:
> 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@linux.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
> > > @@ -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.

split_huge_page set PG_dirty to all subpages unconditionally?
If it's true, yes, it doesn't break MADV_FREE. However, I didn't spot
that piece of code. What I found one is just __split_huge_page_tail
which set PG_dirty to subpage if head page is dirty. IOW, if the head
page is not dirty, tail page will be clean, too.
Could you point out what routine set PG_dirty to all subpages unconditionally?

Thanks.

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