[PATCH] mm/madvise: support both pid and pidfd for process_madvise

[Minchan Kim]

There is a demand[1] to support pid as well pidfd for process_madvise to
reduce unnecessary syscall to get pidfd if the user has control of the
target process(ie, they could guarantee the process is not gone or pid is
not reused).

This patch aims for supporting both options like waitid(2).  So, the
syscall is currently,

        int process_madvise(idtype_t idtype, id_t id, void *addr,
                size_t length, int advice, unsigned long flags);

@which is actually idtype_t for userspace libray and currently, it
supports P_PID and P_PIDFD.

[1]  https://lore.kernel.org/linux-mm/9d849087-3359-c4ab-fbec-859e8186c509@xxxxxxxxxxxxx/

Link: http://lkml.kernel.org/r/20200302193630.68771-6-minchan@xxxxxxxxxx
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
Suggested-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Christian Brauner <christian@xxxxxxxxxx>
Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
Cc: Brian Geffon <bgeffon@xxxxxxxxxx>
Cc: Daniel Colascione <dancol@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: John Dias <joaodias@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Oleksandr Natalenko <oleksandr@xxxxxxxxxx>
Cc: Sandeep Patil <sspatil@xxxxxxxxxx>
Cc: SeongJae Park <sj38.park@xxxxxxxxx>
Cc: SeongJae Park <sjpark@xxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Sonny Rao <sonnyrao@xxxxxxxxxx>
Cc: Tim Murray <timmurray@xxxxxxxxxx>
Cc: <linux-man@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>