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

On Mon 27-02-17 08:10:24, Shaohua Li wrote:

On Mon, Feb 27, 2017 at 03:35:34PM +0100, Michal Hocko wrote:

quoted

On Fri 24-02-17 13:31:45, Shaohua Li wrote:

quoted

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 <redacted>
Cc: Mel Gorman <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Shaohua Li <redacted>

Anyway, feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>

quoted

Looks good to me.
[...]

quoted

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?

Ohh, I figured out when seeing later patch in the series, I then wanted
to get back to this one but forgot... This on its own didn't really tell
me much. Maybe a comment would be helpful or even drop the VM_BUG_ON
altogether.
-- 
Michal Hocko
SUSE Labs

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