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Date: Tue, 23 Apr 2024 07:53:24 +0000
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From: "Yajun Deng" <yajun.deng@linux.dev>
Message-ID: <3c452d5db5b3d5879160ab62a9e0ac4481a6298a@linux.dev>
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Subject: Re: [PATCH] mm/rmap: remove unnecessary page_table_lock
To: "David Hildenbrand" <david@redhat.com>, akpm@linux-foundation.org
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org
In-Reply-To: <b848c431-deca-42e4-925c-673b3fa1f251@redhat.com>
References: <20240422105212.1485788-1-yajun.deng@linux.dev>
 <b848c431-deca-42e4-925c-673b3fa1f251@redhat.com>

April 22, 2024 at 7:24 PM, "David Hildenbrand" <david@redhat.com> wrote:


>=20
>=20On 22.04.24 12:52, Yajun Deng wrote:
>=20
>=20>=20
>=20> page_table_lock is a lock that for page table, we won't change page
> >=20
>=20>  table in __anon_vma_prepare(). As we can see, it works well in
> >=20
>=20>  anon_vma_clone(). They do the same operation.
> >=20
>=20
> We are reusing mm->page_table_lock to serialize, not the *actual* low-l=
evel page table locks that really protect PTEs.
>=20
>=20With that locking gone, there would be nothing protection vma->anon_v=
ma.
>=20
>=20Note that anon_vma_clone() is likely called with the mmap_lock held i=
n write mode, which is not the case for __anon_vma_prepare() ...

Yes, anon_vma_clone() is called with the mmap_lock held. I added mmap_ass=
ert_write_locked(dst->vm_mm) to prove it.
I added mmap_assert_write_locked(vma->vm_mm) in __anon_vma_prepare() at t=
he same time, it shows __anon_vma_prepare()
is also called with the mmap_lock held too.

>=20
>=20I think this change is wrong.
>=20
>=20-- Cheers,
>=20
>=20David / dhildenb
>