On Mon, Aug 30, 2021 at 12:03 AM Rolf Eike Beer [off-list ref] wrote:

Am Freitag, 27. August 2021, 21:18:58 CEST schrieb Suren Baghdasaryan:

quoted

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.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm_types.h |  9 ++++++++-
 mm/madvise.c             | 42 +++++++++++++++++++++++++++++++++-------
 2 files changed, 43 insertions(+), 8 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index bc029f3fca6a..32ac5dc5ebf3 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c

@@ -63,6 +63,27 @@ static int madvise_need_mmap_write(int behavior)
      }
 }

+static struct anon_vma_name *anon_vma_name_alloc(const char *name)
+{
+     struct anon_vma_name *anon_name;
+     size_t len = strlen(name);
+
+     /* Add 1 for NUL terminator at the end of the anon_name->name */
+     anon_name = kzalloc(sizeof(*anon_name) + len + 1,
+                         GFP_KERNEL);
+     kref_init(&anon_name->kref);
+     strcpy(anon_name->name, name);
+
+     return anon_name;
+}

Given that you overwrite anything in that struct anyway this could be reduced
to kmalloc(), no? And it definitely needs a NULL check.

Ack. I'll address both points in the next revision.
Thanks!
Suren.

Eike
--
Rolf Eike Beer, emlix GmbH, http://www.emlix.com
Fon +49 551 30664-0, Fax +49 551 30664-11
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