Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230803172652.2849981-1-surenb@google.com> <20230803172652.2849981-6-surenb@google.com>
 <CAHk-=wiCrWAoEesBuoGoqqufvesicbGp3cX0LyKgEvsFaZNpDA@mail.gmail.com> <20230803181520.yd5ao45rm3rxnsbs@revolver>
In-Reply-To: <20230803181520.yd5ao45rm3rxnsbs@revolver>
From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 3 Aug 2023 11:26:43 -0700
Message-ID: <CAJuCfpHkTNrJhmQABEvEXBJd-Y0yNz+VUPn+ZX5OKHwQiwCr5A@mail.gmail.com>
Subject: Re: [PATCH v3 5/6] mm: always lock new vma before inserting into vma tree
To:     "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Suren Baghdasaryan <surenb@google.com>,
        akpm@linux-foundation.org, jannh@google.com, willy@infradead.org,
        david@redhat.com, peterx@redhat.com, ldufour@linux.ibm.com,
        vbabka@suse.cz, michel@lespinasse.org, jglisse@google.com,
        mhocko@suse.com, hannes@cmpxchg.org, dave@stgolabs.net,
        hughd@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        stable@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Thu, Aug 3, 2023 at 11:15=E2=80=AFAM Liam R. Howlett <Liam.Howlett@oracl=
e.com> wrote:
>
> * Linus Torvalds <torvalds@linux-foundation.org> [230803 14:02]:
> > On Thu, 3 Aug 2023 at 10:27, Suren Baghdasaryan <surenb@google.com> wro=
te:
> > >
> > > While it's not strictly necessary to lock a newly created vma before
> > > adding it into the vma tree (as long as no further changes are perfor=
med
> > > to it), it seems like a good policy to lock it and prevent accidental
> > > changes after it becomes visible to the page faults. Lock the vma bef=
ore
> > > adding it into the vma tree.
> >
> > So my main reaction here is that I started to wonder about the vma allo=
cation.
> >
> > Why doesn't vma_init() do something like
> >
> >         mmap_assert_write_locked(mm);
> >         vma->vm_lock_seq =3D mm->mm_lock_seq;
> >
> > and instead we seem to expect vma_lock_alloc() to do this (and do it
> > very badly indeed).
> >
> > Strange.
> >
> > Anyway, this observation was just a reaction to that "not strictly
> > necessary to lock a newly created vma" part of the commentary. I feel
> > like we could/should just make sure that all newly created vma's are
> > always simply created write-locked.
> >
>
> I thought the same thing initially, but Suren pointed out that it's not
> necessary to hold the vma lock to allocate a vma object.  And it seems
> there is at least one user (arch/ia64/mm/init.c) which does allocate
> outside the lock during ia64_init_addr_space(), which is fine but I'm
> not sure it gains much to do it this way - the insert needs to take the
> lock anyways and it is hardly going to be contended.

Yeah, I remember discussing that. At the time of VMA creation the
mmap_lock might not be write-locked, so mmap_assert_write_locked()
would trigger and mm->mm_lock_seq is not stable. Maybe we can
necessitate holding mmap_lock at the time of VMA creation but that
sounds like an unnecessary restriction. IIRC some drivers also create
vm_are_structs without holding mmap_lock... I'll double-check.

>
> Anywhere else besides an address space setup would probably introduce a
> race.
>
> Thanks,
> Liam
>