Re: [PATCH V5 2/6] mm: don't assume anonymous pages have SwapBacked flag

[Shaohua Li <shli@fb.com>]

On Mon, Feb 27, 2017 at 03:35:34PM +0100, Michal Hocko wrote:
> On Fri 24-02-17 13:31:45, Shaohua Li wrote:
> > There are a few places the code assumes anonymous pages should have
> > SwapBacked flag set. MADV_FREE pages are anonymous pages but we are
> > going to add them to LRU_INACTIVE_FILE list and clear SwapBacked flag
> > for them. The assumption doesn't hold any more, so fix them.
> > 
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Minchan Kim <minchan@kernel.org>
> > Cc: Hugh Dickins <hughd@google.com>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Mel Gorman <mgorman@techsingularity.net>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Shaohua Li <shli@fb.com>
> 
> Looks good to me.
> [...]
> > index 96eb85c..c621088 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1416,7 +1416,8 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >  			 * Store the swap location in the pte.
> >  			 * See handle_pte_fault() ...
> >  			 */
> > -			VM_BUG_ON_PAGE(!PageSwapCache(page), page);
> > +			VM_BUG_ON_PAGE(!PageSwapCache(page) && PageSwapBacked(page),
> > +				page);
> 
> just this part makes me scratch my head. I really do not understand what
> kind of problem it tries to prevent from, maybe I am missing something
> obvoious...

Just check a page which isn't lazyfree but wrongly enters here without swap
entry. Or maybe you suggest we delete this statement?

Thanks,
Shaohua