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([2a02:6b6f:e752:9400:18cf:c773:ee86:c436]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-483207286bcsm123461875e9.12.2026.02.07.10.08.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 07 Feb 2026 10:08:34 -0800 (PST) Message-ID: <687c7173-31c9-457a-9900-68e7f38688ed@gmail.com> Date: Sat, 7 Feb 2026 18:08:33 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: map maximum pages possible in finish_fault To: Dev Jain , akpm@linux-foundation.org, david@kernel.org Cc: lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, kirill@shutemov.name, willy@infradead.org References: <20260206135648.38164-1-dev.jain@arm.com> Content-Language: en-GB From: Usama Arif In-Reply-To: <20260206135648.38164-1-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit > @@ -5619,49 +5619,53 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > nr_pages = folio_nr_pages(folio); > > /* Using per-page fault to maintain the uffd semantics */ > - if (unlikely(userfaultfd_armed(vma)) || unlikely(needs_fallback)) { > + if (unlikely(userfaultfd_armed(vma)) || unlikely(single_page_fallback)) { > nr_pages = 1; > } else if (nr_pages > 1) { > - pgoff_t idx = folio_page_idx(folio, page); > - /* The page offset of vmf->address within the VMA. */ > - pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff; > - /* The index of the entry in the pagetable for fault page. */ > - pgoff_t pte_off = pte_index(vmf->address); > + > + /* Ensure mapping stays within VMA and PMD boundaries */ > + unsigned long pmd_boundary_start = ALIGN_DOWN(vmf->address, PMD_SIZE); > + unsigned long pmd_boundary_end = pmd_boundary_start + PMD_SIZE; > + unsigned long va_of_folio_start = vmf->address - ((vmf->pgoff - folio->index) * PAGE_SIZE); > + unsigned long va_of_folio_end = va_of_folio_start + nr_pages * PAGE_SIZE; > + unsigned long end_addr; Hello! Can va_of_folio_start underflow here? For e.g. if you MAP_FIXED at a very low address and vmf->pgoff is big. max3() would then pick this huge value as start_addr/ I think the old code guarded against this explicitly below: if (unlikely(vma_off < idx || ...)) { nr_pages = 1; } > + > + start_addr = max3(vma->vm_start, pmd_boundary_start, va_of_folio_start); > + end_addr = min3(vma->vm_end, pmd_boundary_end, va_of_folio_end); > > /* > - * Fallback to per-page fault in case the folio size in page > - * cache beyond the VMA limits and PMD pagetable limits. > + * Do not allow to map with PTEs across i_size to preserve > + * SIGBUS semantics. > + * > + * Make an exception for shmem/tmpfs that for long time > + * intentionally mapped with PMDs across i_size. > */ > - if (unlikely(vma_off < idx || > - vma_off + (nr_pages - idx) > vma_pages(vma) || > - pte_off < idx || > - pte_off + (nr_pages - idx) > PTRS_PER_PTE)) { > - nr_pages = 1; > - } else { > - /* Now we can set mappings for the whole large folio. */ > - addr = vmf->address - idx * PAGE_SIZE; > - page = &folio->page; > - } > + if (mapping && !shmem_mapping(mapping)) > + end_addr = min(end_addr, va_of_folio_start + (file_end - folio->index) * PAGE_SIZE); > + > + nr_pages = (end_addr - start_addr) >> PAGE_SHIFT; > + page = folio_page(folio, (start_addr - va_of_folio_start) >> PAGE_SHIFT); > } > > vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, > - addr, &vmf->ptl); > + start_addr, &vmf->ptl); > if (!vmf->pte) > return VM_FAULT_NOPAGE; > > /* Re-check under ptl */ > if (nr_pages == 1 && unlikely(vmf_pte_changed(vmf))) { > - update_mmu_tlb(vma, addr, vmf->pte); > + update_mmu_tlb(vma, start_addr, vmf->pte); > ret = VM_FAULT_NOPAGE; > goto unlock; > } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) { > - needs_fallback = true; > + single_page_fallback = true; > + try_pmd_mapping = false; > pte_unmap_unlock(vmf->pte, vmf->ptl); > goto fallback; > } > > folio_ref_add(folio, nr_pages - 1); > - set_pte_range(vmf, folio, page, nr_pages, addr); > + set_pte_range(vmf, folio, page, nr_pages, start_addr); > type = is_cow ? MM_ANONPAGES : mm_counter_file(folio); > add_mm_counter(vma->vm_mm, type, nr_pages); > ret = 0;