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Date:   Wed, 6 Oct 2021 19:39:40 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Colin Cross <ccross@google.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Michal Hocko <mhocko@suse.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Randy Dunlap <rdunlap@infradead.org>,
        Kalesh Singh <kaleshsingh@google.com>,
        Peter Xu <peterx@redhat.com>, rppt@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        vincenzo.frascino@arm.com,
        Chinwen Chang (=?UTF-8?Q?=E5=BC=B5=E9=8C=A6?= =?UTF-8?Q?=E6=96=87?=) 
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Subject: Re: [PATCH v10 2/3] mm: add a field to store names for private
 anonymous memory
Message-Id: <20211006193940.c261f21fcd14b4b52aae1fbc@linux-foundation.org>
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        <20211001160830.700c36b32b736478000b3420@linux-foundation.org>
        <CAJuCfpGpMru4z=ZMezRQW56tHNjrWHU3jWhG3qzuXvuUytq-3w@mail.gmail.com>
        <CAJuCfpFWUXQ445VcqTcV1kNY3AWX=wB5iaeDAX_=+xZefjTUjg@mail.gmail.com>
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On Mon, 4 Oct 2021 09:21:42 -0700 Suren Baghdasaryan <surenb@google.com> wrote:

> > > > The name pointers are not shared between vmas even if they contain the
> > > > same name. The name pointer is stored in a union with fields that are
> > > > only used on file-backed mappings, so it does not increase memory usage.
> > > >
> > > > The patch is based on the original patch developed by Colin Cross, more
> > > > specifically on its latest version [1] posted upstream by Sumit Semwal.
> > > > It used a userspace pointer to store vma names. In that design, name
> > > > pointers could be shared between vmas. However during the last upstreaming
> > > > attempt, Kees Cook raised concerns [2] about this approach and suggested
> > > > to copy the name into kernel memory space, perform validity checks [3]
> > > > and store as a string referenced from vm_area_struct.
> > > > One big concern is about fork() performance which would need to strdup
> > > > anonymous vma names. Dave Hansen suggested experimenting with worst-case
> > > > scenario of forking a process with 64k vmas having longest possible names
> > > > [4]. I ran this experiment on an ARM64 Android device and recorded a
> > > > worst-case regression of almost 40% when forking such a process. This
> > > > regression is addressed in the followup patch which replaces the pointer
> > > > to a name with a refcounted structure that allows sharing the name pointer
> > > > between vmas of the same name. Instead of duplicating the string during
> > > > fork() or when splitting a vma it increments the refcount.
> > >
> > > Generally, the patch adds a bunch of code which a lot of users won't
> > > want.  Did we bust a gut to reduce this impact?  Was a standalone
> > > config setting considered?
> >
> > I didn't consider a standalone config for this feature because when
> > not used it has no memory impact at runtime. As for the image size, I
> > built Linus' ToT with and without this patchset with allmodconfig and

allnoconfig would be more interesting.  People who want small kernels
won't be using allmodconfig!

> > the sizes are:
> > Without the patchset:
> > $ size vmlinux
> >    text    data     bss     dec     hex filename
> > 40763556 58424519 29016228 128204303 7a43e0f vmlinux
> >
> > With the patchset:
> > $ size vmlinux
> >    text    data     bss     dec     hex filename
> > 40765068 58424671 29016228 128205967 7a4448f vmlinux
> >
> > The increase seems quite small, so I'm not sure if it warrants a
> > separate config option.
> 
> Andrew, do you still think we need a separate CONFIG option? I fixed
> the build issue when CONFIG_ADVISE_SYSCALLS=n and would like to post
> the update but if you want to have a separate config then I can post
> that together with the fix. Please let me know.

I don't see much downside to the standalone option.  More complexity
for developers/testers, I guess.  But such is life?