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From:   Rolf Eike Beer <eb@emlix.com>
To:     akpm@linux-foundation.org, Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH v8 3/3] mm: add anonymous vma name refcounting
Date:   Mon, 30 Aug 2021 09:03:37 +0200
Message-ID: <15537178.k4V9gYNSIy@devpool47>
Organization: emlix GmbH
In-Reply-To: <20210827191858.2037087-4-surenb@google.com>
References: <20210827191858.2037087-1-surenb@google.com> <20210827191858.2037087-4-surenb@google.com>
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From: Rolf Eike Beer <eb@emlix.com>
To: akpm@linux-foundation.org, Suren Baghdasaryan <surenb@google.com>
Cc: ccross@google.com, sumit.semwal@linaro.org, mhocko@suse.com, dave.hansen@intel.com, keescook@chromium.org, willy@infradead.org, kirill.shutemov@linux.intel.com, vbabka@suse.cz, hannes@cmpxchg.org, corbet@lwn.net, viro@zeniv.linux.org.uk, rdunlap@infradead.org, kaleshsingh@google.com, peterx@redhat.com, rppt@kernel.org, peterz@infradead.org, catalin.marinas@arm.com, vincenzo.frascino@arm.com, chinwen.chang@mediatek.com, axelrasmussen@google.com, aarcange@redhat.com, jannh@google.com, apopple@nvidia.com, jhubbard@nvidia.com, yuzhao@google.com, will@kernel.org, fenghua.yu@intel.com, thunder.leizhen@huawei.com, hughd@google.com, feng.tang@intel.com, jgg@ziepe.ca, guro@fb.com, tglx@linutronix.de, krisman@collabora.com, chris.hyser@oracle.com, pcc@google.com, ebiederm@xmission.com, axboe@kernel.dk, legion@kernel.org, songmuchun@bytedance.com, viresh.kumar@linaro.org, thomascedeno@google.com, sashal@kernel.org, cxfcosmos@gmail.com, linux@rasmusvillemoes.dk, linux-kernel@vger.kernel.org,
  linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, kernel-team@android.com, surenb@google.com
Subject: Re: [PATCH v8 3/3] mm: add anonymous vma name refcounting
Date: Mon, 30 Aug 2021 09:03:37 +0200
Message-ID: <15537178.k4V9gYNSIy@devpool47>
Organization: emlix GmbH
In-Reply-To: <20210827191858.2037087-4-surenb@google.com>
References: <20210827191858.2037087-1-surenb@google.com> <20210827191858.2037087-4-surenb@google.com>

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.
>=20
> 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(-)
>=20
> 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)
>  	}
>  }
>=20
> +static struct anon_vma_name *anon_vma_name_alloc(const char *name)
> +{
> +	struct anon_vma_name *anon_name;
> +	size_t len =3D strlen(name);
> +
> +	/* Add 1 for NUL terminator at the end of the anon_name->name */
> +	anon_name =3D 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 reduc=
ed=20
to kmalloc(), no? And it definitely needs a NULL check.

Eike
=2D-=20
Rolf Eike Beer, emlix GmbH, http://www.emlix.com
=46on +49 551 30664-0, Fax +49 551 30664-11
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