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

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Message-ID: <15537178.k4V9gYNSIy@devpool47>
Date:   Mon, 30 Aug 2021 09:03:37 +0200
From:   Rolf Eike Beer <eb@...ix.com>
To:     akpm@...ux-foundation.org, Suren Baghdasaryan <surenb@...gle.com>
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Subject: Re: [PATCH v8 3/3] mm: add anonymous vma name refcounting

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@...gle.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.

Eike
-- 
Rolf Eike Beer, emlix GmbH, http://www.emlix.com
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