Re: [PATCH] mm: write protect MADV_FREE pages

[Minchan Kim <minchan@kernel.org>]

Hello,

On Wed, Jan 25, 2017 at 09:15:19AM -0800, Shaohua Li wrote:
> On Tue, Jan 24, 2017 at 11:32:12AM +0900, Minchan Kim wrote:
> > Hi Shaohua,
> > 
> > On Mon, Jan 23, 2017 at 03:15:52PM -0800, Shaohua Li wrote:
> > > The page reclaim has an assumption writting to a page with clean pte
> > > should trigger a page fault, because there is a window between pte zero
> > > and tlb flush where a new write could come. If the new write doesn't
> > > trigger page fault, page reclaim will not notice it and think the page
> > > is clean and reclaim it. The MADV_FREE pages don't comply with the rule
> > > and the pte is just cleaned without writeprotect, so there will be no
> > > pagefault for new write. This will cause data corruption.
> > 
> > It's hard to understand.
> > Could you show me exact scenario seqence you have in mind?
> Sorry for the delay, for some reason, I didn't receive the mail.
> in try_to_unmap_one:
> CPU 1:						CPU2:
> 1. pteval = ptep_get_and_clear(mm, address, pte);
> 2.						write to the address
> 3. tlb flush
> 
> step 1 will get a clean pteval, step2 dirty it, but the unmap missed the dirty
> bit so discard the page without pageout. step2 doesn't trigger a page fault,

I thought about that when Mel introduced deferred flush and concluded it
should be no problem from theses discussion:
 
1. https://lkml.org/lkml/2015/4/15/565
2. https://lkml.org/lkml/2015/4/16/136

So, shouldn't it make trap?

Ccing Mel.

> because the tlb cache still has the pte entry. The defer flush makes the window
> bigger actually. There are comments about this in try_to_unmap_one too.



> 
> Thanks,
> Shaohua
> 
> > > Cc: Minchan Kim <minchan@kernel.org>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Hugh Dickins <hughd@google.com>
> > > Cc: Rik van Riel <riel@surriel.com>
> > > Cc: stable@kernel.org
> > > Signed-off-by: Shaohua Li <shli@fb.com>
> > > ---
> > >  mm/huge_memory.c | 1 +
> > >  mm/madvise.c     | 1 +
> > >  2 files changed, 2 insertions(+)
> > > 
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 9a6bd6c..9cc5de5 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -1381,6 +1381,7 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > >  			tlb->fullmm);
> > >  		orig_pmd = pmd_mkold(orig_pmd);
> > >  		orig_pmd = pmd_mkclean(orig_pmd);
> > > +		orig_pmd = pmd_wrprotect(orig_pmd);
> > >  
> > >  		set_pmd_at(mm, addr, pmd, orig_pmd);
> > >  		tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index 0e3828e..bfb6800 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -373,6 +373,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> > >  
> > >  			ptent = pte_mkold(ptent);
> > >  			ptent = pte_mkclean(ptent);
> > > +			ptent = pte_wrprotect(ptent);
> > >  			set_pte_at(mm, addr, pte, ptent);
> > >  			if (PageActive(page))
> > >  				deactivate_page(page);
> > > -- 
> > > 2.9.3
> > > 
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