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Shutemov" , Hugh Dickins , Axel Rasmussen , Matthew Wilcox , Andrew Morton , Andrea Arcangeli , Nadav Amit Subject: [PATCH RFC 14/30] shmem/userfaultfd: Allow wr-protect none pte for file-backed mem Date: Fri, 15 Jan 2021 12:08:51 -0500 Message-Id: <20210115170907.24498-15-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: File-backed memory differs from anonymous memory in that even if the pte = is missing, the data could still resides either in the file or in page/swap = cache. So when wr-protect a pte, we need to consider none ptes too. We do that by installing the uffd-wp special swap pte as a marker. So wh= en there's a future write to the pte, the fault handler will go the special = path to first fault-in the page as read-only, then report to userfaultfd serve= r with the wr-protect message. On the other hand, when unprotecting a page, it's also possible that the = pte got unmapped but replaced by the special uffd-wp marker. Then we'll need= to be able to recover from a uffd-wp special swap pte into a none pte, so that = the next access to the page will fault in correctly as usual when trigger the= fault handler next time, rather than sending a uffd-wp message. Special care needs to be taken throughout the change_protection_range() process. Since now we allow user to wr-protect a none pte, we need to be= able to pre-populate the page table entries if we see !anonymous && MM_CP_UFFD= _WP requests, otherwise change_protection_range() will always skip when the p= gtable entry does not exist. Note that this patch only covers the small pages (pte level) but not cove= ring any of the transparent huge pages yet. But this will be a base for thps = too. Signed-off-by: Peter Xu --- mm/mprotect.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 48 insertions(+) diff --git a/mm/mprotect.c b/mm/mprotect.c index e75bfe43cedd..c9390fd673fe 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -29,6 +29,7 @@ #include #include #include +#include #include #include #include @@ -176,6 +177,32 @@ static unsigned long change_pte_range(struct vm_area= _struct *vma, pmd_t *pmd, set_pte_at(vma->vm_mm, addr, pte, newpte); pages++; } + } else if (unlikely(is_swap_special_pte(oldpte))) { + if (uffd_wp_resolve && !vma_is_anonymous(vma) && + pte_swp_uffd_wp_special(oldpte)) { + /* + * This is uffd-wp special pte and we'd like to + * unprotect it. What we need to do is simply + * recover the pte into a none pte; the next + * page fault will fault in the page. + */ + pte_clear(vma->vm_mm, addr, pte); + pages++; + } + } else { + /* It must be an none page, or what else?.. */ + WARN_ON_ONCE(!pte_none(oldpte)); + if (unlikely(uffd_wp && !vma_is_anonymous(vma))) { + /* + * For file-backed mem, we need to be able to + * wr-protect even for a none pte! Because + * even if the pte is null, the page/swap cache + * could exist. + */ + set_pte_at(vma->vm_mm, addr, pte, + pte_swp_mkuffd_wp_special(vma)); + pages++; + } } } while (pte++, addr +=3D PAGE_SIZE, addr !=3D end); arch_leave_lazy_mmu_mode(); @@ -209,6 +236,25 @@ static inline int pmd_none_or_clear_bad_unless_trans= _huge(pmd_t *pmd) return 0; } =20 +/* + * File-backed vma allows uffd wr-protect upon none ptes, because even i= f pte + * is missing, page/swap cache could exist. When that happens, the wr-p= rotect + * information will be stored in the page table entries with the marker = (e.g., + * PTE_SWP_UFFD_WP_SPECIAL). Prepare for that by always populating the = page + * tables to pte level, so that we'll install the markers in change_pte_= range() + * where necessary. + * + * Note that we only need to do this in pmd level, because if pmd does n= ot + * exist, it means the whole range covered by the pmd entry (of a pud) d= oes not + * contain any valid data but all zeros. Then nothing to wr-protect. + */ +#define change_protection_prepare(vma, pmd, addr, cp_flags) \ + do { \ + if (unlikely((cp_flags & MM_CP_UFFD_WP) && pmd_none(*pmd) && \ + !vma_is_anonymous(vma))) \ + WARN_ON_ONCE(pte_alloc(vma->vm_mm, pmd)); \ + } while (0) + static inline unsigned long change_pmd_range(struct vm_area_struct *vma, pud_t *pud, unsigned long addr, unsigned long end, pgprot_t newprot, unsigned long cp_flags) @@ -227,6 +273,8 @@ static inline unsigned long change_pmd_range(struct v= m_area_struct *vma, =20 next =3D pmd_addr_end(addr, end); =20 + change_protection_prepare(vma, pmd, addr, cp_flags); + /* * Automatic NUMA balancing walks the tables with mmap_lock * held for read. It's possible a parallel update to occur --=20 2.26.2