From: Carlos Llamas <cmllamas@google.com>
To: Suren Baghdasaryan <surenb@google.com>,
"Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"Liam R. Howlett" <Liam.Howlett@oracle.com>,
Michal Hocko <mhocko@suse.com>,
Guenter Roeck <groeck@chromium.org>,
Douglas Anderson <dianders@chromium.org>,
Christian Brauner <brauner@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
linux-mm@kvack.org
Subject: Re: BUG in binder_vma_close() at mmap_assert_locked() in stable v5.15
Date: Mon, 12 Sep 2022 19:11:10 +0000 [thread overview]
Message-ID: <Yx+ETrzRWGZSIq+m@google.com> (raw)
In-Reply-To: <CAJuCfpE-UirnCEzQGeREGnA08QA2rUwnoB8VNT23+C9Ktwr4+A@mail.gmail.com>
On Fri, Sep 09, 2022 at 01:03:08PM -0700, Suren Baghdasaryan wrote:
> On Fri, Sep 9, 2022 at 12:35 PM Carlos Llamas <cmllamas@google.com> wrote:
> >
> > Does this mean that users of async calls such as find_vma() can't rely
> > on mmap_lock to avoid racing with remove_vma()? I see the following
> > pattern is used quite often:
> >
> > mmap_read_lock(mm);
> > vma = find_vma(mm, addr);
> > [...]
> > mmap_read_unlock(mm);
> >
> > Is this not a real concern? I'd drop the asserts from binder and call it
> > a day. However, we would also need to fix our race with vm_ops->close().
>
> I think by the time exit_mmap() calls remove_vma() there can be no
> other user of that mm to race with, even oom-reaper would have
> finished by then (see:
> https://elixir.bootlin.com/linux/v5.15.67/source/mm/mmap.c#L3157).
> So, generally remove_vma() would be done under mmap_lock write
> protection but in case of exit_mmap() that's not necessary. Michal,
> please correct me if I'm wrong.
I see, that makes more sense.
Then it sounds to me like binder should be using mmget_not_zero() to
serialize against exit_mmap() during these async calls. I'll have a
closer look at this change.
Also, we should drop the mmap_lock asserts in binder from v5.15 as the
expectations there are incorrect. Again, this was done in [1], but for
different reasons. We could simply amend a small note to the commit log
with an accurate reason for the backport.
Liam, wdyt?
[1] https://lore.kernel.org/all/20220829201254.1814484-5-cmllamas@google.com/
next prev parent reply other threads:[~2022-09-12 19:11 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-08 20:28 BUG in binder_vma_close() at mmap_assert_locked() in stable v5.15 Carlos Llamas
2022-09-08 22:33 ` Suren Baghdasaryan
2022-09-09 19:35 ` Carlos Llamas
2022-09-09 20:03 ` Suren Baghdasaryan
2022-09-12 19:11 ` Carlos Llamas [this message]
2022-09-13 6:36 ` Liam Howlett
2022-09-23 20:50 ` Carlos Llamas
2022-09-28 23:21 ` Suren Baghdasaryan
2022-09-29 14:39 ` Carlos Llamas
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