thp: use is_zero_pfn() only after pte_present() check

Use is_zero_pfn() on pteval only after pte_present() check on pteval
(It might be better idea to introduce is_zero_pte() which checks
pte_present() first).

Otherwise when working on a swap or migration entry and if pte_pfn's
result is equal to zero_pfn by chance, we lose user's data in
__collapse_huge_page_copy().  So if you're unlucky, the application
segfaults and finally you could see below message on exit:

BUG: Bad rss-counter state mm:ffff88007f099300 idx:2 val:3

Fixes: ca0984caa823 ("mm: incorporate zero pages into transparent huge pages")
Signed-off-by: Minchan Kim <[email protected]>
Reviewed-by: Andrea Arcangeli <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Mel Gorman <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: <[email protected]>	[4.1+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions