Re: [PATCH 2/7] mm/rmap: map_pte() was not handling private ZONE_DEVICE page properly

[Jerome Glisse]

On Fri, Aug 31, 2018 at 12:05:38AM +1000, Balbir Singh wrote:
> On Fri, Aug 24, 2018 at 03:25:44PM -0400, jglisse@xxxxxxxxxx wrote:
> > From: Ralph Campbell <rcampbell@xxxxxxxxxx>
> > 
> > Private ZONE_DEVICE pages use a special pte entry and thus are not
> > present. Properly handle this case in map_pte(), it is already handled
> > in check_pte(), the map_pte() part was lost in some rebase most probably.
> > 
> > Without this patch the slow migration path can not migrate back private
> > ZONE_DEVICE memory to regular memory. This was found after stress
> > testing migration back to system memory. This ultimatly can lead the
> > CPU to an infinite page fault loop on the special swap entry.
> > 
> > Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> > Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >  mm/page_vma_mapped.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > index ae3c2a35d61b..1cf5b9bfb559 100644
> > --- a/mm/page_vma_mapped.c
> > +++ b/mm/page_vma_mapped.c
> > @@ -21,6 +21,15 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw)
> >  			if (!is_swap_pte(*pvmw->pte))
> >  				return false;
> >  		} else {
> > +			if (is_swap_pte(*pvmw->pte)) {
> > +				swp_entry_t entry;
> > +
> > +				/* Handle un-addressable ZONE_DEVICE memory */
> > +				entry = pte_to_swp_entry(*pvmw->pte);
> > +				if (is_device_private_entry(entry))
> > +					return true;
> > +			}
> > +
> 
> This happens just for !PVMW_SYNC && PVMW_MIGRATION? I presume this
> is triggered via the remove_migration_pte() code path? Doesn't
> returning true here imply that we've taken the ptl lock for the
> pvmw?

This happens through try_to_unmap() from migrate_vma_unmap() and thus
has !PVMW_SYNC and !PVMW_MIGRATION

But you are right about the ptl lock, so looking at code we were just
doing pte modification without holding the pte lock but the
page_vma_mapped_walk() would not try to unlock as pvmw->ptl == NULL
so this never triggered any warning.

I am gonna post a v2 shortly which address that.

Cheers,
Jérôme