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Shutemov" , Hugh Dickins , Axel Rasmussen , Matthew Wilcox , Andrew Morton , Andrea Arcangeli , Nadav Amit Subject: [PATCH RFC 16/30] shmem/userfaultfd: Handle the left-overed special swap ptes Date: Fri, 15 Jan 2021 12:08:53 -0500 Message-Id: <20210115170907.24498-17-peterx@redhat.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20210115170907.24498-1-peterx@redhat.com> References: <20210115170907.24498-1-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" 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 000b457ad087..3537a43b69c9 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -331,6 +331,21 @@ 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 de45333626f7..9947bcf92663 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2456,7 +2456,18 @@ static int shmem_mfill_atomic_pte(struct mm_struct= *dst_mm, 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 lru_cache_add(page); --=20 2.26.2