Re: [PATCH v10 2/3] mm: add a field to store names for private anonymous memory

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Oct 6, 2021 at 7:39 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> 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!

Sure, I will check that and report back.

>
> > > 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?

Sounds good to me. I will post a new version with a separate config if
we get over the objections of using numbers instead of strings.
Thanks!

>
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