mm: ksm: do not block on page lock when searching stable tree

mm: ksm: do not block on page lock when searching stable tree

ksmd needs to search the stable tree to look for a suitable KSM page, but
the KSM page might be locked for a long time due to the KSM page rmap
walk.

It is not worth waiting for the lock; the page can be skipped and we can
then try to merge it in the next scan to avoid long stalls if its content
is still intact.

Introduce an async mode to get_ksm_page() to not block on the page lock,
as try_to_merge_one_page() does.

Return -EBUSY if the trylock fails, since a NULL means failure to find a
suitable KSM page, which is a valid case.

Link: http://lkml.kernel.org/r/1541618201-120667-2-git-send-email-yang.shi@linux.alibaba.com
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>