On 25.04.22 15:27, Miaohe Lin wrote:
> When non-lru movable page was freed from under us, __ClearPageMovable must
> have been done. Even if it's not done, ClearPageIsolated here won't hurt
> as page will be freed anyway. So we can thus remove unneeded lock page and
> PageMovable check here.
>
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> ---
> mm/migrate.c | 8 ++------
> 1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index b779646665fe..0fc4651b3e39 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1093,12 +1093,8 @@ static int unmap_and_move(new_page_t get_new_page,
> /* page was freed from under us. So we are done. */
> ClearPageActive(page);
> ClearPageUnevictable(page);
> - if (unlikely(__PageMovable(page))) {
> - lock_page(page);
> - if (!PageMovable(page))
> - ClearPageIsolated(page);
> - unlock_page(page);
> - }
> + if (unlikely(__PageMovable(page)))
> + ClearPageIsolated(page);
> goto out;
> }
Hm, that code+change raises a couple of questions.
We're doing here the same as in putback_movable_pages(). So I guess the
difference here is that the caller did release the reference while the
page was isolated, while we don't assume the same in
putback_movable_pages().
Shouldn't whoever owned the page have cleared that? IOW, is it even
valid that we see a movable or isolated page here (WARN/BUG?)?
At least for balloon compaction, I remember that __PageMovable() is
properly cleared before freeing it via balloon_page_delete().
Also, I am not sure how reliable that page count check is here: if we'd
have another speculative reference to the page, we might see
"page_count(page) > 1" and not take that path, although the previous
owner released the last reference.
--
Thanks,
David / dhildenb
