Re: [PATCH] mm: fix special swap entry handling on copy mm

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Mon, Aug 12, 2013 at 05:49:37PM -0400, Jerome Glisse wrote:
> On Mon, Aug 12, 2013 at 05:36:40PM -0400, Naoya Horiguchi wrote:
> > Hi Jerome,
> > 
> > On Mon, Aug 12, 2013 at 11:43:24AM -0400, j.glisse@gmail.com wrote:
> > > From: Jerome Glisse <jglisse@redhat.com>
> > > 
> > > Prior to this copy_one_pte will never reach the special swap file
> > > handling code because swap_duplicate will return invalid value.
> > > 
> > > Note this is not fatal so nothing bad ever happen because of that.
> > > Reason is that copy_pte_range would break of its loop and call
> > > add_swap_count_continuation which would see its a special swap
> > > file and return 0 triggering copy_pte_range to try again. Because
> > > we try again there is a huge chance that the temporarily special
> > > migration pte is now again valid and pointing to a new valid page.
> > > 
> > > This patch just split handling of special swap entry from regular
> > > one inside copy_one_pte.
> > > 
> > > (Note i spotted that while reading code i haven't tested my theory.)
> > > 
> > > Signed-off-by: Jerome Glisse <jglisse@redhat.com>
> > 
> > non_swap_entry() means not only migration entry, but also hwpoison entry,
> > so it seems to me that simply moving the swap_duplicate() into the
> > if(!non_swap_entry) block can change the behavior for hwpoison entry.
> > Would it be nice to add check for such a case?
> > 
> > Thanks,
> > Naoya Horiguchi
> 
> Well if my reading of the code is right for hwpoison entry current code will
> loop indefinitly inside the kernel on fork if one entry is set to hwpoison.

(Sorry if I missed something, but) __swap_duplicate always returns
-EINVAL if non_swap_entry is true and then swap_duplicate returns 0.
So copy_one_pte() doesn't return at if (swap_duplicate(entry) < 0)
block for non_swap_entry. So ...

> > > Prior to this copy_one_pte will never reach the special swap file
> > > handling code because swap_duplicate will return invalid value.

this seems not correct to me.
Could you explain more about this point?
(Maybe I don't understand the terminology "special swap file"...)

> My patch does not handle hwpoison because it seems useless as there is nothing
> to do for hwpoison pte beside giving setting the new pte to hwpoison to. So
> the fork child will also have a pte with hwpoison. My patch do just that.

Yes, just copying hwpoison entry looks a right behavior to me.

Thanks,
Naoya Horiguchi

> So change in behavior is current kernel loop indefinitly in kernel with hwpoison
> pte on fork, vs child get hwpoison pte with my patch. Meaning that both child
> and father can live as long as they dont access the hwpoisoned ptes.
> 
> > 
> > > ---
> > >  mm/memory.c | 26 +++++++++++++-------------
> > >  1 file changed, 13 insertions(+), 13 deletions(-)
> > > 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 1ce2e2a..9f907dd 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -833,20 +833,20 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > >  		if (!pte_file(pte)) {
> > >  			swp_entry_t entry = pte_to_swp_entry(pte);
> > >  
> > > -			if (swap_duplicate(entry) < 0)
> > > -				return entry.val;
> > > -
> > > -			/* make sure dst_mm is on swapoff's mmlist. */
> > > -			if (unlikely(list_empty(&dst_mm->mmlist))) {
> > > -				spin_lock(&mmlist_lock);
> > > -				if (list_empty(&dst_mm->mmlist))
> > > -					list_add(&dst_mm->mmlist,
> > > -						 &src_mm->mmlist);
> > > -				spin_unlock(&mmlist_lock);
> > > -			}
> > > -			if (likely(!non_swap_entry(entry)))
> > > +			if (likely(!non_swap_entry(entry))) {
> > > +				if (swap_duplicate(entry) < 0)
> > > +					return entry.val;
> > > +
> > > +				/* make sure dst_mm is on swapoff's mmlist. */
> > > +				if (unlikely(list_empty(&dst_mm->mmlist))) {
> > > +					spin_lock(&mmlist_lock);
> > > +					if (list_empty(&dst_mm->mmlist))
> > > +						list_add(&dst_mm->mmlist,
> > > +							 &src_mm->mmlist);
> > > +					spin_unlock(&mmlist_lock);
> > > +				}
> > >  				rss[MM_SWAPENTS]++;
> > > -			else if (is_migration_entry(entry)) {
> > > +			} else if (is_migration_entry(entry)) {
> > >  				page = migration_entry_to_page(entry);
> > >  
> > >  				if (PageAnon(page))
> > > -- 
> > > 1.8.3.1
> > > 
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> 
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