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Tue, 02 Jan 2024 08:58:45 -0800 (PST) MIME-Version: 1.0 References: <20231230025607.2476912-1-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Tue, 2 Jan 2024 08:58:33 -0800 Message-ID: Subject: Re: [PATCH 1/1] userfaultfd: fix move_pages_pte() splitting folio under RCU read lock To: Peter Xu Cc: akpm@linux-foundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, david@redhat.com, ryan.roberts@arm.com, hughd@google.com, mhocko@suse.com, axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com, jannh@google.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-Rspam-User: X-Stat-Signature: ucoc3d7cwx1fwazwgunmypdqd4fijk6d X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: DA2B71C001E X-HE-Tag: 1704214726-554808 X-HE-Meta: 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 W0e2U7no cdt2Xpj1YgSxscsZ2dNSXkoCWAvaclmi4EmjoyioRZYKjcSiBytkpP+aKOMvGjeY8NVVCsJJzmQ1cLUsn17ElzPjntBUjN9BYahL2pU6U7XTkxLLeJBzET7GLJpqi0h/RIWPqyGfn+r+MqJD5KsE2N3Qr4LpxT92JGaUiG4f2HpibFxBHT5GwGfkvFJBhSVJMP2ynlvYJ289guF0WIqfiFMR1MY8UA8oiH6pMv+XM9kqrAcXKQJYdjSjxMJO37V8x5a2hnTnTHSqVPIK6hBYYk7u01xflI9IrOLTGxBvPExQIbpA= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 Tue, Jan 2, 2024 at 1:00=E2=80=AFAM Peter Xu wrote: > > On Fri, Dec 29, 2023 at 06:56:07PM -0800, Suren Baghdasaryan wrote: > > @@ -1078,9 +1078,14 @@ static int move_pages_pte(struct mm_struct *mm, = pmd_t *dst_pmd, pmd_t *src_pmd, > > > > /* at this point we have src_folio locked */ > > if (folio_test_large(src_folio)) { > > + /* split_folio() can block */ > > + pte_unmap(&orig_src_pte); > > + pte_unmap(&orig_dst_pte); > > + src_pte =3D dst_pte =3D NULL; > > err =3D split_folio(src_folio); > > if (err) > > goto out; > > + goto retry; > > } > > Do we also need to clear src_folio and src_folio_pte? If the folio is a > thp, I think it means it's pte mapped here. Then after the split we may > want to fetch the small folio after the split, not the head one? I think we need to re-fetch the src_folio only if the src_addr falls into a non-head page. Looking at the __split_huge_page(), the head page is skipped in the last loop, so I think it should stay valid. That said, maybe it's just an implementation detail of the __split_huge_page() and I should not rely on that and refetch anyway? > > -- > Peter Xu >