On Wed, Mar 2, 2022 at 5:50 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > 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); Nice idea! Reviewed-by: Yang Shi <shy828301@xxxxxxxxx> > + > if (PageSwapBacked(page)) > __mod_lruvec_page_state(page, NR_SHMEM_PMDMAPPED, > nr_pages);
