mm: add anonymous vma name refcounting

While forking a process with high number (64K) of named anonymous vmas
the overhead caused by strdup() is noticeable.  Experiments with ARM64
Android device show up to 40% performance regression when forking a
process with 64k unpopulated anonymous vmas using the max name lengths
vs the same process with the same number of anonymous vmas having no
name.

Introduce anon_vma_name refcounted structure to avoid the overhead of
copying vma names during fork() and when splitting named anonymous vmas.

When a vma is duplicated, instead of copying the name we increment the
refcount of this structure.  Multiple vmas can point to the same
anon_vma_name as long as they increment the refcount.  The name member
of anon_vma_name structure is assigned at structure allocation time and
is never changed.  If vma name changes then the refcount of the original
structure is dropped, a new anon_vma_name structure is allocated to hold
the new name and the vma pointer is updated to point to the new
structure.

With this approach the fork() performance regressions is reduced 3-4x
times and with usecases using more reasonable number of VMAs (a few
thousand) the regressions is not measurable.

Link: https://lkml.kernel.org/r/20211019215511.3771969-3-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Colin Cross <ccross@google.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jan Glauber <jan.glauber@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rob Landley <rob@landley.net>
Cc: "Serge E. Hallyn" <serge.hallyn@ubuntu.com>
Cc: Shaohua Li <shli@fusionio.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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