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Mon, 05 Feb 2024 14:24:25 -0800 (PST) MIME-Version: 1.0 References: <20240129193512.123145-1-lokeshgidra@google.com> <20240129193512.123145-4-lokeshgidra@google.com> <20240129203626.uq5tdic4z5qua5qy@revolver> <20240130025803.2go3xekza5qubxgz@revolver> <20240131214104.rgw3x5vuap43xubi@revolver> <20240205220022.a4qy7xlv6jpcsnh7@revolver> In-Reply-To: <20240205220022.a4qy7xlv6jpcsnh7@revolver> From: Lokesh Gidra Date: Mon, 5 Feb 2024 14:24:13 -0800 Message-ID: Subject: Re: [PATCH v2 3/3] userfaultfd: use per-vma locks in userfaultfd operations To: "Liam R. Howlett" , Lokesh Gidra , Suren Baghdasaryan , akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, selinux@vger.kernel.org, kernel-team@android.com, aarcange@redhat.com, peterx@redhat.com, david@redhat.com, axelrasmussen@google.com, bgeffon@google.com, willy@infradead.org, jannh@google.com, kaleshsingh@google.com, ngeoffray@google.com, timmurray@google.com, rppt@kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 3179918000C X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: qi5ss7xem76wh9x93dm8txw3byemc7k5 X-HE-Tag: 1707171866-785656 X-HE-Meta: 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 oCjjQmoy 0Vk7OC+ynDOKgT8o5CWAkU1yg7xS2losV5H5ygWt7tNTWwAR9+PN2WQteiBiJ4Aa8UUa5IlHJcVQIA5RSOSqXovPkvpxefClC94n7QIuZvphbTGjAqfmIdIlGyL++ncZfVCVdTsthFk5MF0ZG6TjiwUNtDuKEBA2jYbNqg5031TGXNQnDkuotyrfw/xtXxaJkLo7nHbxtFifN118bazBxKCrcS5CvjxPcDxAjAAn++djH+ZrYFPGTyqD92MalcVMnhqj99owBATthYH/M28C2RTaEQg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Feb 5, 2024 at 2:00=E2=80=AFPM Liam R. Howlett wrote: > > * Lokesh Gidra [240205 16:55]: > ... > > > > > > We can take care of anon_vma as well here right? I can take a boo= l > > > > > parameter ('prepare_anon' or something) and then: > > > > > > > > > > if (vma) { > > > > > if (prepare_anon && vma_is_anonymous(vma)) && > > > > > !anon_vma_prepare(vma)) { > > > > > vma =3D ERR_PTR(-ENOMEM); > > > > > goto out_unlock; > > > > > } > > > > > > vma_aquire_read_lock(vma); > > > > > } > > > > > out_unlock: > > > > > > mmap_read_unlock(mm); > > > > > > return vma; > > > > > > } > > > > > > > > Do you need this? I didn't think this was happening in the code as > > > > written? If you need it I would suggest making it happen always an= d > > > > ditch the flag until a user needs this variant, but document what's > > > > going on in here or even have a better name. > > > > > > I think yes, you do need this. I can see calls to anon_vma_prepare() > > > under mmap_read_lock() protection in both mfill_atomic_hugetlb() and > > > in mfill_atomic(). This means, just like in the pagefault path, we > > > modify vma->anon_vma under mmap_read_lock protection which guarantees > > > that adjacent VMAs won't change. This is important because > > > __anon_vma_prepare() uses find_mergeable_anon_vma() that needs the > > > neighboring VMAs to be stable. Per-VMA lock guarantees stability of > > > the VMA we locked but not of its neighbors, therefore holding per-VMA > > > lock while calling anon_vma_prepare() is not enough. The solution > > > Lokesh suggests would call anon_vma_prepare() under mmap_read_lock an= d > > > therefore would avoid the issue. > > > > > ... > > > anon_vma_prepare() is also called in validate_move_areas() via move_pag= es(). > > Probably worth doing it unconditionally and have a comment as to why it > is necessary. > The src_vma (in case of move_pages()) doesn't need to have it. The only reason I'm not inclined to make it unconditional is what if some future user of lock_vma() doesn't need it for their purpose? Why allocate anon_vma in that case. > Does this avoid your locking workaround? Not sure which workaround you are referring to. I am almost done implementing your suggestion. Very soon will share the next version of the patch-set. > > Thanks, > Liam