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Wed, 27 Sep 2023 15:49:15 -0700 (PDT) MIME-Version: 1.0 References: <20230923013148.1390521-1-surenb@google.com> <20230923013148.1390521-3-surenb@google.com> In-Reply-To: From: Jann Horn Date: Thu, 28 Sep 2023 00:48:37 +0200 Message-ID: Subject: Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, peterx@redhat.com, david@redhat.com, hughd@google.com, mhocko@suse.com, axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com, zhangpeng362@huawei.com, bgeffon@google.com, kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: qph5n3mecuft9hor54jwnn5mw1p417kf X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 95A3440007 X-Rspam-User: X-HE-Tag: 1695854956-705426 X-HE-Meta: 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 p/lqDg+M fbjaTlcGwVfFXegTX9FVWBd2U4W3TY1X76Pb9YLssWkJIREMUnSxnzGwTBSbNMlhLXg8ulK0mtQZQV3RkZsxDFzybwIxkk1zQINRnL1s3c5+P+GObSOm0QSsDVxCxSEMBd9Mf5fv8FaBGAL8t8wla/hraAb4Jd+vXw954Q81iX97YGoSju15qMyTBoVwcAYKC1+tUmFapCsivIFEVRjqHO2f4mweaNAf3tA3jGJQFs7dtsnztNAv+OPdRZu8d8xR1HFybLTrIUxdGAGTBPtxz8MvEVpnhO8hBlPl031uDddWDiDBfR/EUCsl9Haxn16lAhugNE5Qe6nlbN9m52R8hi4yNJw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000025, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 27, 2023 at 11:08=E2=80=AFPM Suren Baghdasaryan wrote: > On Wed, Sep 27, 2023 at 1:42=E2=80=AFPM Suren Baghdasaryan wrote: > > > > On Wed, Sep 27, 2023 at 1:04=E2=80=AFPM Jann Horn wr= ote: > > > > > > On Wed, Sep 27, 2023 at 8:08=E2=80=AFPM Suren Baghdasaryan wrote: > > > > On Wed, Sep 27, 2023 at 5:47=E2=80=AFAM Jann Horn wrote: > > > > > On Sat, Sep 23, 2023 at 3:31=E2=80=AFAM Suren Baghdasaryan wrote: > > > > > > + dst_pmdval =3D pmdp_get_lockless(dst_pmd); > > > > > > + /* > > > > > > + * If the dst_pmd is mapped as THP don't overri= de it and just > > > > > > + * be strict. If dst_pmd changes into TPH after= this check, the > > > > > > + * remap_pages_huge_pmd() will detect the chang= e and retry > > > > > > + * while remap_pages_pte() will detect the chan= ge and fail. > > > > > > + */ > > > > > > + if (unlikely(pmd_trans_huge(dst_pmdval))) { > > > > > > + err =3D -EEXIST; > > > > > > + break; > > > > > > + } > > > > > > + > > > > > > + ptl =3D pmd_trans_huge_lock(src_pmd, src_vma); > > > > > > + if (ptl && !pmd_trans_huge(*src_pmd)) { > > > > > > + spin_unlock(ptl); > > > > > > + ptl =3D NULL; > > > > > > + } > > > > > > > > > > This still looks wrong - we do still have to split_huge_pmd() > > > > > somewhere so that remap_pages_pte() works. > > > > > > > > Hmm, I guess this extra check is not even needed... > > > > > > Hm, and instead we'd bail at the pte_offset_map_nolock() in > > > remap_pages_pte()? I guess that's unusual but works... > > > > Yes, that's what I was thinking but I agree, that seems fragile. Maybe > > just bail out early if (ptl && !pmd_trans_huge())? > > No, actually we can still handle is_swap_pmd() case by splitting it > and remapping the individual ptes. So, I can bail out only in case of > pmd_devmap(). FWIW I only learned today that "real" swap PMDs don't actually exist - only migration entries, which are encoded as swap PMDs, exist. You can see that when you look through the cases that something like __split_huge_pmd() or zap_pmd_range() actually handles. So I think if you wanted to handle all the PMD types properly here without splitting, you could do that without _too_ much extra code. But idk if it's worth it.