PageDoubleMap is maintained differently for anon and for shmem+file: the shmem+file one was never cleared, because a safe place to do so could not be found; so it would blight future use of the cached hugepage until evicted. See https://lore.kernel.org/lkml/1571938066-29031-1-git-send-email-yang.shi@xxxxxxxxxxxxxxxxx/ But page_add_file_rmap() does provide a safe place to do so (though later than one might wish): allowing testing to return to an initial state without a damaging drop_caches. Fixes: 9a73f61bdb8a ("thp, mlock: do not mlock PTE-mapped file huge pages") Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> --- mm/rmap.c | 11 +++++++++++ 1 file changed, 11 insertions(+) --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1251,6 +1251,17 @@ void page_add_file_rmap(struct page *page, } if (!atomic_inc_and_test(compound_mapcount_ptr(page))) goto out; + + /* + * It is racy to ClearPageDoubleMap in page_remove_file_rmap(); + * but page lock is held by all page_add_file_rmap() compound + * callers, and SetPageDoubleMap below warns if !PageLocked: + * so here is a place that DoubleMap can be safely cleared. + */ + VM_WARN_ON_ONCE(!PageLocked(page)); + if (nr == nr_pages && PageDoubleMap(page)) + ClearPageDoubleMap(page); + if (PageSwapBacked(page)) __mod_lruvec_page_state(page, NR_SHMEM_PMDMAPPED, nr_pages);
