On Thu, Oct 24, 2019 at 10:28 AM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote: > > File THP sets PageDoubleMap flag when the first it gets PTE mapped, but > the flag is never cleared until the THP is freed. This result in > unbalanced state although it is not a big deal. > > Clear the flag when the last compound_mapcount is gone. It should be > cleared when all the PTE maps are gone (become PMD mapped only) as well, > but this needs check all subpage's _mapcount every time any subpage's > rmap is removed, the overhead may be not worth. The anonymous THP also > just clears PageDoubleMap flag when the last PMD map is gone. > > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> Looks good to me. Thanks! Acked-by: Song Liu <songliubraving@xxxxxx> > --- > Hugh thought it is unnecessary to fix it completely due to the overhead > (https://lkml.org/lkml/2019/10/22/1011), but it sounds simple to achieve > the similar balance as anonymous THP. > > mm/rmap.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/rmap.c b/mm/rmap.c > index 0c7b2a9..d17cbf3 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1236,6 +1236,9 @@ static void page_remove_file_rmap(struct page *page, bool compound) > __dec_node_page_state(page, NR_SHMEM_PMDMAPPED); > else > __dec_node_page_state(page, NR_FILE_PMDMAPPED); > + > + /* The last PMD map is gone */ > + ClearPageDoubleMap(compound_head(page)); > } else { > if (!atomic_add_negative(-1, &page->_mapcount)) > goto out; > -- > 1.8.3.1 > >
