Re: [PATCH v8 3/3] mm: add anonymous vma name refcounting

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Aug 30, 2021 at 12:03 AM Rolf Eike Beer <eb@emlix.com> wrote:
>
> Am Freitag, 27. August 2021, 21:18:58 CEST schrieb Suren Baghdasaryan:
> > 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
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