Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Oct 12, 2021 at 10:01 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Tue, Oct 12, 2021 at 12:44 AM David Hildenbrand <david@redhat.com> wrote:
> >
> > > I'm still evaluating the proposal to use memfds but I'm not sure if
> > > the issue that David Hildenbrand mentioned about additional memory
> > > consumed in pagecache (which has to be addressed) is the only one we
> > > will encounter with this approach. If anyone knows of any potential
> > > issues with using memfds as named anonymous memory, I would really
> > > appreciate your feedback before I go too far in that direction.
> >
> > [MAP_PRIVATE memfd only behave that way with 4k, not with huge pages, so
> > I think it just has to be fixed. It doesn't make any sense to allocate a
> > page for the pagecache ("populate the file") when accessing via a
> > private mapping that's supposed to leave the file untouched]
> >
> > My gut feeling is if you really need a string as identifier, then try
> > going with memfds. Yes, we might hit some road blocks to be sorted out,
> > but it just logically makes sense to me: Files have names. These names
> > exist before mapping and after mapping. They "name" the content.
>
> I'm investigating this direction. I don't have much background with
> memfds, so I'll need to digest the code first.

I've done some investigation into the possibility of using memfds to
name anonymous VMAs. Here are my findings:

1. Forking a process with anonymous vmas named using memfd is 5-15%
slower than with prctl (depends on the number of VMAs in the process
being forked). Profiling shows that i_mmap_lock_write() dominates
dup_mmap(). Exit path is also slower by roughly 9% with
free_pgtables() and fput() dominating exit_mmap(). Fork performance is
important for Android because almost all processes are forked from
zygote, therefore this limitation already makes this approach
prohibitive.

2. mremap() usage to grow the mapping has an issue when used with memfds:

fd = memfd_create(name, MFD_ALLOW_SEALING);
ftruncate(fd, size_bytes);
ptr = mmap(NULL, size_bytes, prot, MAP_PRIVATE, fd, 0);
close(fd);
ptr = mremap(ptr, size_bytes, size_bytes * 2, MREMAP_MAYMOVE);
touch_mem(ptr, size_bytes * 2);

This would generate a SIGBUS in touch_mem(). I believe it's because
ftruncate() specified the size to be size_bytes and we are accessing
more than that after remapping. prctl() does not have this limitation
and we do have a usecase for growing a named VMA.

3. Leaves an fd exposed, even briefly, which may lead to unexpected
flaws (e.g. anything using mmap MAP_SHARED could allow exposures or
overwrites). Even MAP_PRIVATE, if an attacker writes into the file
after ftruncate() and before mmap(), can cause private memory to be
initialized with unexpected data.

4. There is a usecase in the Android userspace where vma naming
happens after memory was allocated. Bionic linker does in-memory
relocations and then names some relocated sections.

In the light of these findings, could the current patchset be reconsidered?
Thanks,
Suren.


>
> >
> > Maybe it's just me, but the whole interface, setting the name via a
> > prctl after the mapping was already instantiated doesn't really spark
> > joy at my end. That's not a strong pushback, but if we can avoid it
> > using something that's already there, that would be very much preferred.
>
> Actually that's one of my worries about using memfds. There might be
> cases when we need to name a vma after it was mapped. memfd_create()
> would not allow us to do that AFAIKT. But I need to check all usages
> to say if that's really an issue.
> Thanks!
>
> >
> > --
> > Thanks,
> >
> > David / dhildenb
> >
> > --
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> >