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

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Date:   Fri, 27 Aug 2021 22:28:41 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
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Subject: Re: [PATCH v8 3/3] mm: add anonymous vma name refcounting

On Fri, Aug 27, 2021 at 12:18:58PM -0700, Suren Baghdasaryan wrote:
> 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.

I like the refcounting; thank you!

Since patch2 adds a lot of things that are changed by patch3; maybe
combine them?

-- 
Kees Cook