Re: [RFC 04/11] mm: remove SWAP_MLOCK check for SWAP_SUCCESS in ttu

Kirill A. Shutemov
2017-03-07T14:26:48Z

On Thu, Mar 02, 2017 at 03:39:18PM +0900, Minchan Kim wrote:
> If the page is mapped and rescue in ttuo, page_mapcount(page) == 0 cannot
> be true so page_mapcount check in ttu is enough to return SWAP_SUCCESS.
> IOW, SWAP_MLOCK check is redundant so remove it.
>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> mm/rmap.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 3a14013..0a48958 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1523,7 +1523,7 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
> else
> ret = rmap_walk(page, &rwc);
>
> - if (ret != SWAP_MLOCK && !page_mapcount(page))
> + if (!page_mapcount(page))

Hm. I think there's bug in current code.
It should be !total_mapcount(page) otherwise it can be false-positive if
there's THP mapped with PTEs.

And in this case ret != SWAP_MLOCK is helpful to cut down some cost.
Althouth it should be fine to remove it, I guess.

--
Kirill A. Shutemov