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[2003:cb:c72a:9100:23d2:3800:cdcc:90f0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43d43f47196sm77012285e9.16.2025.03.21.06.17.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 21 Mar 2025 06:17:14 -0700 (PDT) Message-ID: <69256f08-1d1c-45ac-945a-495ef4ffc558@redhat.com> Date: Fri, 21 Mar 2025 14:17:12 +0100 Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 3/3] kernel/events/uprobes: uprobe_write_opcode() rewrite To: Jiri Olsa Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Andrew Morton , Andrii Nakryiko , Matthew Wilcox , Russell King , Masami Hiramatsu , Oleg Nesterov , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Ian Rogers , Adrian Hunter , "Liang, Kan" , Tong Tiangen References: <20250321113713.204682-1-david@redhat.com> <20250321113713.204682-4-david@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 21.03.25 14:05, Jiri Olsa wrote: > On Fri, Mar 21, 2025 at 12:37:13PM +0100, David Hildenbrand wrote: > > SNIP > >> +static int __uprobe_write_opcode(struct vm_area_struct *vma, >> + struct folio_walk *fw, struct folio *folio, >> + unsigned long opcode_vaddr, uprobe_opcode_t opcode) >> +{ >> + const unsigned long vaddr = opcode_vaddr & PAGE_MASK; >> + const bool is_register = !!is_swbp_insn(&opcode); >> + bool pmd_mappable; >> + >> + /* For now, we'll only handle PTE-mapped folios. */ >> + if (fw->level != FW_LEVEL_PTE) >> + return -EFAULT; >> + >> + /* >> + * See can_follow_write_pte(): we'd actually prefer a writable PTE here, >> + * but the VMA might not be writable. >> + */ >> + if (!pte_write(fw->pte)) { >> + if (!PageAnonExclusive(fw->page)) >> + return -EFAULT; >> + if (unlikely(userfaultfd_pte_wp(vma, fw->pte))) >> + return -EFAULT; >> + /* SOFTDIRTY is handled via pte_mkdirty() below. */ >> + } >> + >> + /* >> + * We'll temporarily unmap the page and flush the TLB, such that we can >> + * modify the page atomically. >> + */ >> + flush_cache_page(vma, vaddr, pte_pfn(fw->pte)); >> + fw->pte = ptep_clear_flush(vma, vaddr, fw->ptep); >> + copy_to_page(fw->page, opcode_vaddr, &opcode, UPROBE_SWBP_INSN_SIZE); >> + >> + /* >> + * When unregistering, we may only zap a PTE if uffd is disabled and >> + * there are no unexpected folio references ... >> + */ >> + if (is_register || userfaultfd_missing(vma) || >> + (folio_ref_count(folio) != folio_mapcount(folio) + 1 + >> + folio_test_swapcache(folio) * folio_nr_pages(folio))) >> + goto remap; >> + >> + /* >> + * ... and the mapped page is identical to the original page that >> + * would get faulted in on next access. >> + */ >> + if (!orig_page_is_identical(vma, vaddr, fw->page, &pmd_mappable)) >> + goto remap; >> + >> + dec_mm_counter(vma->vm_mm, MM_ANONPAGES); >> + folio_remove_rmap_pte(folio, fw->page, vma); >> + if (!folio_mapped(folio) && folio_test_swapcache(folio) && >> + folio_trylock(folio)) { >> + folio_free_swap(folio); >> + folio_unlock(folio); >> + } >> + folio_put(folio); > > hi, > it's probably ok and I'm missing something, but why do we call folio_put > in here, I'd think it's done by folio_put call in uprobe_write_opcode Hi! We're zapping a page table mapping mapping (folio_remove_rmap_pte()), so we have to drop that reference. That's the folio_put(old_folio) in the old __replace_page(). Thanks! -- Cheers, David / dhildenb