From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 8FC2730F531 for ; Tue, 10 Feb 2026 13:29:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770730165; cv=none; b=T47968ABZy9XNioSdZjqWS+vPySrc2MNBvdWTYvnsARkIrBKpIkvxgp+RA6tg+4hkpebpLUazADihcmDBlBXErCAv4E8FE86F6QUHTLjkanMrHDHW+82IqVLUcDQQn4+wQD/duAdeZZwTqLT1YWJjRYpgKQKP7OrC7bhiZrOAoo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770730165; c=relaxed/simple; bh=o65xhzEOZ/6EMaDBakb6KTOI1NSm+gMpWzOCZZ6LFc4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kYKQaonpRDkyqzAHMdmrOg+1VxU0PKsO/y6M+vCoyvL8O/qhufiMWRz2IlfZXkG417iS7eY65XcsMNffRutzcTx8TbdRb3ub6hxaUT8zOr3qc1W/JqsFluEzZ/odExmf/gPIY48i95g6mJIiqYWnJWFhYQyPDaIwsCbZGDvexjM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9370B339; Tue, 10 Feb 2026 05:29:16 -0800 (PST) Received: from [10.164.19.61] (unknown [10.164.19.61]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7AAC93F63F; Tue, 10 Feb 2026 05:29:18 -0800 (PST) Message-ID: Date: Tue, 10 Feb 2026 18:59:15 +0530 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: Usama Arif , 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> <687c7173-31c9-457a-9900-68e7f38688ed@gmail.com> Content-Language: en-US From: Dev Jain In-Reply-To: <687c7173-31c9-457a-9900-68e7f38688ed@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 07/02/26 11:38 pm, Usama Arif wrote: >> @@ -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; > } Indeed! Thanks for the spot, I'll fix this. > >> + >> + 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;