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Shutemov" , Hugh Dickins , Axel Rasmussen , Matthew Wilcox , Andrew Morton , Andrea Arcangeli , Nadav Amit Subject: [PATCH RFC 06/30] mm/userfaultfd: Introduce special pte for unmapped file-backed mem Date: Fri, 15 Jan 2021 12:08:43 -0500 Message-Id: <20210115170907.24498-7-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: This patch introduces a very special swap-like pte for file-backed memori= es. Currently it's only defined for x86_64 only, but as long as any arch that= can properly define the UFFD_WP_SWP_PTE_SPECIAL value as requested, it should conceptually work too. We will use this special pte to arm the ptes that got either unmapped or swapped out for a file-backed region that was previously wr-protected. T= his special pte could trigger a page fault just like swap entries, and as lon= g as the page fault will satisfy pte_none()=3D=3Dfalse && pte_present()=3D=3Df= alse. Then we can revive the special pte into a normal pte backed by the page c= ache. This idea is greatly inspired by Hugh and Andrea in the discussion, which= is referenced in the links below. The other idea (from Hugh) is that we use swp_type=3D=3D1 and swp_offset=3D= 0 as the special pte. The current solution (as pointed out by Andrea) is slightly preferred in that we don't even need swp_entry_t knowledge at all in trap= ping these accesses. Meanwhile, we also reuse _PAGE_SWP_UFFD_WP from the anon= ymous swp entries. This patch only introduces the special pte and its operators. It's not y= et applied to have any functional difference. Link: https://lore.kernel.org/lkml/20201126222359.8120-1-peterx@redhat.co= m/ Link: https://lore.kernel.org/lkml/20201130230603.46187-1-peterx@redhat.c= om/ Suggested-by: Andrea Arcangeli Suggested-by: Hugh Dickins Signed-off-by: Peter Xu --- arch/x86/include/asm/pgtable.h | 28 ++++++++++++++++++++++++++++ include/asm-generic/pgtable_uffd.h | 3 +++ include/linux/userfaultfd_k.h | 21 +++++++++++++++++++++ 3 files changed, 52 insertions(+) diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtabl= e.h index a02c67291cfc..379bae343dd1 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -1329,6 +1329,34 @@ static inline pmd_t pmd_swp_clear_soft_dirty(pmd_t= pmd) #endif =20 #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP + +/* + * This is a very special swap-like pte that marks this pte as "wr-prote= cted" + * by userfaultfd-wp. It should only exist for file-backed memory where= the + * page (previously got wr-protected) has been unmapped or swapped out. + * + * For anonymous memories, the userfaultfd-wp _PAGE_SWP_UFFD_WP bit is k= ept + * along with a real swp entry instead. + * + * Let's make some rules for this special pte: + * + * (1) pte_none()=3D=3Dfalse, so that it'll not trigger a missing page f= ault. + * + * (2) pte_present()=3D=3Dfalse, so that it's recognized as swap (is_swa= p_pte). + * + * (3) pte_swp_uffd_wp()=3D=3Dtrue, so it can be tested just like a swap= pte that + * contains a valid swap entry, so that we can check a swap pte alwa= ys + * using "is_swap_pte() && pte_swp_uffd_wp()" without caring about w= hether + * there's one swap entry inside of the pte. + * + * (4) It should not be a valid swap pte anywhere, so that when we see t= his pte + * we know it does not contain a swap entry. + * + * For x86, the simplest special pte which satisfies all of above should= be the + * pte with only _PAGE_SWP_UFFD_WP bit set (where swp_type=3D=3Dswp_offs= et=3D=3D0). + */ +#define UFFD_WP_SWP_PTE_SPECIAL __pte(_PAGE_SWP_UFFD_WP) + static inline pte_t pte_swp_mkuffd_wp(pte_t pte) { return pte_set_flags(pte, _PAGE_SWP_UFFD_WP); diff --git a/include/asm-generic/pgtable_uffd.h b/include/asm-generic/pgt= able_uffd.h index 828966d4c281..95e9811ce9d1 100644 --- a/include/asm-generic/pgtable_uffd.h +++ b/include/asm-generic/pgtable_uffd.h @@ -2,6 +2,9 @@ #define _ASM_GENERIC_PGTABLE_UFFD_H =20 #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP + +#define UFFD_WP_SWP_PTE_SPECIAL __pte(0) + static __always_inline int pte_uffd_wp(pte_t pte) { return 0; diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.= h index a8e5f3ea9bb2..7d6071a65ded 100644 --- a/include/linux/userfaultfd_k.h +++ b/include/linux/userfaultfd_k.h @@ -98,6 +98,17 @@ extern int userfaultfd_unmap_prep(struct vm_area_struc= t *vma, extern void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf); =20 +static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma= ) +{ + WARN_ON_ONCE(vma_is_anonymous(vma)); + return UFFD_WP_SWP_PTE_SPECIAL; +} + +static inline bool pte_swp_uffd_wp_special(pte_t pte) +{ + return pte_same(pte, UFFD_WP_SWP_PTE_SPECIAL); +} + #else /* CONFIG_USERFAULTFD */ =20 /* mm helpers */ @@ -182,6 +193,16 @@ static inline void userfaultfd_unmap_complete(struct= mm_struct *mm, { } =20 +static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma= ) +{ + return __pte(0); +} + +static inline bool pte_swp_uffd_wp_special(pte_t pte) +{ + return false; +} + #endif /* CONFIG_USERFAULTFD */ =20 #endif /* _LINUX_USERFAULTFD_K_H */ --=20 2.26.2