Date:   Mon, 23 Jan 2023 17:55:49 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        akpm@linux-foundation.org, michel@lespinasse.org,
        jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org,
        mgorman@techsingularity.net, dave@stgolabs.net,
        peterz@infradead.org, ldufour@linux.ibm.com,
        laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org,
        songliubraving@fb.com, peterx@redhat.com, david@redhat.com,
        dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de,
        kent.overstreet@linux.dev, punit.agrawal@bytedance.com,
        lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com,
        axelrasmussen@google.com, joelaf@google.com, minchan@google.com,
        jannh@google.com, shakeelb@google.com, tatashin@google.com,
        edumazet@google.com, gthelen@google.com, gurua@google.com,
        arjunroy@google.com, soheil@google.com, hughlynch@google.com,
        leewalsh@google.com, posk@google.com, linux-mm@kvack.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free
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On Mon 23-01-23 08:22:53, Suren Baghdasaryan wrote:
> On Mon, Jan 23, 2023 at 1:56 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Fri 20-01-23 09:50:01, Suren Baghdasaryan wrote:
> > > On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox <willy@infradead.org> wrote:
> > [...]
> > > > The page fault handler (or whatever other reader -- ptrace, proc, etc)
> > > > should have a refcount on the mm_struct, so we can't be in this path
> > > > trying to free VMAs.  Right?
> > >
> > > Hmm. That sounds right. I checked process_mrelease() as well, which
> > > operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps
> > > VMAs without freeing them, so we are still good. Michal, do you agree
> > > this is ok?
> >
> > Don't we need RCU procetions for the vma life time assurance? Jann has
> > already shown how rwsem is not safe wrt to unlock and free without RCU.
> 
> Jann's case requires a thread freeing the VMA to be blocked on vma
> write lock waiting for the vma real lock to be released by a page
> fault handler. However exit_mmap() means mm->mm_users==0, which in
> turn suggests that there are no racing page fault handlers and no new
> page fault handlers will appear. Is that a correct assumption? If so,
> then races with page fault handlers can't happen while in exit_mmap().
> Any other path (other than page fault handlers), accesses vma->lock
> under protection of mmap_lock (for read or write, does not matter).
> One exception is when we operate on an isolated VMA, then we don't
> need mmap_lock protection, but exit_mmap() does not deal with isolated
> VMAs, so out of scope here. exit_mmap() frees vm_area_structs under
> protection of mmap_lock in write mode, so races with anything other
> than page fault handler should be safe as they are today.

I do not see you talking about #PF (RCU + vma read lock protected) with
munmap. It is my understanding that the latter will synchronize over per
vma lock (along with mmap_lock exclusive locking). But then we are back
to the lifetime guarantees, or do I miss anything.
-- 
Michal Hocko
SUSE Labs