Re: [PATCH v8 2/3] mm: add a field to store names for private anonymous memory

[Suren Baghdasaryan <surenb@google.com>]

On Sat, Aug 28, 2021 at 2:28 PM Cyrill Gorcunov <gorcunov@gmail.com> wrote:
>
> On Fri, Aug 27, 2021 at 12:18:57PM -0700, Suren Baghdasaryan wrote:
> >
> > The name is stored in a pointer in the shared union in vm_area_struct
> > that points to a null terminated string. Anonymous vmas with the same
> > name (equivalent strings) and are otherwise mergeable will be merged.
> > 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.
> >
> > [1] https://lore.kernel.org/linux-mm/20200901161459.11772-4-sumit.semwal@linaro.org/
> > [2] https://lore.kernel.org/linux-mm/202009031031.D32EF57ED@keescook/
> > [3] https://lore.kernel.org/linux-mm/202009031022.3834F692@keescook/
> > [4] https://lore.kernel.org/linux-mm/5d0358ab-8c47-2f5f-8e43-23b89d6a8e95@intel.com/
> ...
> > +
> > +/* mmap_lock should be read-locked */
> > +static inline bool is_same_vma_anon_name(struct vm_area_struct *vma,
> > +                                      const char *name)
> > +{
> > +     const char *vma_name = vma_anon_name(vma);
> > +
> > +     if (likely(!vma_name))
> > +             return name == NULL;
> > +
> > +     return name && !strcmp(name, vma_name);
> > +}
>
> Hi Suren! There is very important moment with this new feature: if
> we assign a name to some VMA it won't longer be mergeable even if
> near VMA matches by all other attributes such as flags, permissions
> and etc. I mean our vma_merge() start considering the vma namings
> and names mismatch potentially blocks merging which happens now
> without this new feature. Is it known behaviour or I miss something
> pretty obvious here?

Hi Cyrill,
Correct, this is a known drawback of naming an anonymous VMA. I think
I'll need to document this in prctl(2) manpage, which I should update
to include this new PR_SET_VMA_ANON_NAME option.
Thanks for pointing it out!
Suren.