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/[email protected]
Signed-off-by: Suren Baghdasaryan <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Colin Cross <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jan Glauber <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rob Landley <[email protected]>
Cc: "Serge E. Hallyn" <[email protected]>
Cc: Shaohua Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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