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[174.91.135.175]) by smtp.gmail.com with ESMTPSA id n6sm5031793qtx.22.2021.03.22.17.49.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 22 Mar 2021 17:49:36 -0700 (PDT) From: Peter Xu To: linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: "Kirill A . Shutemov" , Jerome Glisse , Mike Kravetz , Matthew Wilcox , Andrew Morton , Axel Rasmussen , Hugh Dickins , peterx@redhat.com, Nadav Amit , Andrea Arcangeli , Mike Rapoport Subject: [PATCH 13/23] shmem/userfaultfd: Handle the left-overed special swap ptes Date: Mon, 22 Mar 2021 20:49:02 -0400 Message-Id: <20210323004912.35132-14-peterx@redhat.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20210323004912.35132-1-peterx@redhat.com> References: <20210323004912.35132-1-peterx@redhat.com> MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="US-ASCII" X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 88016E0011C9 X-Stat-Signature: qjmoyh9qwtdrwdcpbwjm5po16xnciceg Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf13; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616460579-774471 Content-Transfer-Encoding: quoted-printable 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: Note that the special uffd-wp swap pte can be left over even if the page = under the pte got evicted. Normally when evict a page, we will unmap the ptes = by walking through the reverse mapping. However we never tracked such infor= mation for the special swap ptes because they're not real mappings but just mark= ers. So we need to take care of that when we see a marker but when it's actual= ly meaningless (the page behind it got evicted). We have already taken care of that in e.g. alloc_set_pte() where we'll tr= eat the special swap pte as pte_none() when necessary. However we need to al= so teach userfaultfd itself on either UFFDIO_COPY or handling page faults, s= o that everything will still work as expected. Signed-off-by: Peter Xu --- fs/userfaultfd.c | 15 +++++++++++++++ mm/shmem.c | 13 ++++++++++++- 2 files changed, 27 insertions(+), 1 deletion(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index bd83379d4dd2..72956f9cc892 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -329,6 +329,21 @@ static inline bool userfaultfd_must_wait(struct user= faultfd_ctx *ctx, */ if (pte_none(*pte)) ret =3D true; + /* + * We also treat the swap special uffd-wp pte as the pte_none() here. + * This should in most cases be a missing event, as we never handle + * wr-protect upon a special uffd-wp swap pte - it should first be + * converted into a normal read request before handling wp. It just + * means the page/swap cache that backing this pte is gone, so this + * special pte is leftover. + * + * We can't simply replace it with a none pte because we're not with + * the pgtable lock here. Instead of taking it and clearing the pte, + * the easy way is to let UFFDIO_COPY understand this pte too when + * trying to install a new page onto it. + */ + if (pte_swp_uffd_wp_special(*pte)) + ret =3D true; if (!pte_write(*pte) && (reason & VM_UFFD_WP)) ret =3D true; pte_unmap(pte); diff --git a/mm/shmem.c b/mm/shmem.c index e88aaabaeb27..90d67406af66 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2469,7 +2469,18 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_m= m, pmd_t *dst_pmd, goto out_release_unlock; =20 ret =3D -EEXIST; - if (!pte_none(*dst_pte)) + /* + * Besides the none pte, we also allow UFFDIO_COPY to install a pte + * onto the uffd-wp swap special pte, because that pte should be the + * same as a pte_none() just in that it contains wr-protect information + * (which could only be dropped when unmap the memory). + * + * It's safe to drop that marker because we know this is part of a + * MISSING fault, and the caller is very clear about this page missing + * rather than wr-protected. Then we're sure the wr-protect bit is + * just a leftover so it's useless already. + */ + if (!pte_none(*dst_pte) && !pte_swp_uffd_wp_special(*dst_pte)) goto out_release_unlock; =20 if (!is_continue) { --=20 2.26.2