From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F36283B71A4; Wed, 26 Aug 2026 07:57:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787731033; cv=none; b=m8UJSJWmTDgxW0PCGT4HWOrfFj+6iKpCE1903VhM+DkvzRljX0hhJEKa9kqwPPnp3aayDGh/8YociMRH+DgFK60Pq/kgSMqAIYr1yLe0Trlc+0R1B6DSuDp1TB9IJC6N4lCiY59amWkI82xJk5F1d1w5+0QfbTy0z6LrmFrl6BI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787731033; c=relaxed/simple; bh=tyTC6W1xuyX6MjmkZ31a7cq0HgAHdRsB7Y3erRGg5OI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mTk2gvwoYfWeCZ5Z5/NEXspSa5UAfcGWjftl3r8rHZYt2bfIlOCtgvOM/UbxhyFVTeoVvn0IVamxYIsE0Gqj7NLi9eT9Mxtu/7RqnSN7CgMMiQEtk9g9bACQAQ0kKG/Kr1OHAqN1PeVXWyGYPU+HNK8yTzJk8a/S3Dpi7IKmHqU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kn+BsycT; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kn+BsycT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EEECF1F00A3A; Wed, 26 Aug 2026 07:57:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787731031; bh=JoSpfgQVWPrL1xC+VY8JnkH+r+BJbUIhnLuY0YtzjCA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kn+BsycT04gQ8Z7v7bg784woMyrj3i89HvLmRjTDXrcv9qjJqmPSbDnGUXZGCzLJ/ ie+ALYgFBja7ljDjCXCtXqjGKwBfbhr8XyuW42PRidc7g/D4PI/5yKDesjbZ7RSGdI +7tAytW8JomdXUvRk1Al0ZRYjnre/arA5xMA6W3Z1w9mmHSnMIvOgAW1OhAgasnwGF o9Q6xGSEYUFB3Iv1Hr9ml/HGlawCYbvkrSqCs+vW8Fdh8I27a56Ahin2NnFUm4j2CA smzIOP7F8Kfry1m4/mH4sxuJgHxvnTgd5J5OFdxdUHE18sw971vI52NqMJFNukphKk 51PMIoH44i1Ag== Message-ID: Date: Wed, 26 Aug 2026 09:57:05 +0200 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 v3 1/3] mm: khugepaged: fix swap entry value to folio_pfn() To: Vernon Yang Cc: akpm@linux-foundation.org, ljs@kernel.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, zokeefe@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org, Vernon Yang References: <20260824092935.73892-1-vernon2gm@gmail.com> <20260824092935.73892-2-vernon2gm@gmail.com> <6a9c2369-5589-4f2a-bcfe-c6e3b46a1ccd@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/26/26 04:44, Vernon Yang wrote: > On Mon, Aug 24, 2026 at 01:54:18PM +0200, David Hildenbrand (Arm) wrote: >> On 8/24/26 11:29, Vernon Yang wrote: >>> From: Vernon Yang >>> >>> When the swap entries found exceed max_ptes_swap, the loop is left via >>> break with folio still holding the xarray value that encodes the swap >>> entry, not valid folio pointer. >>> >>> That value is passed to trace_mm_khugepaged_scan_file(), which feeds it >>> to folio_pfn(). On FLATMEM and SPARSEMEM_VMEMMAP, the page_to_pfn() is >>> plain pointer arithmetic, so the trace event merely prints bogus >>> scan_pfn. On classic SPARSEMEM, the page_to_pfn() reads page->flags, >>> dereferencing the tiny encoded integer and oopsing khugepaged whenever >>> the trace event is enabled. >>> >>> So when folio is the swap entry value, simply set pfn to -1, just like >>> exhausted scan naturally. >>> >>> And the folio_put() has maybe dropped the last reference of folio. The >>> trace_mm_khugepaged_scan_file() is left with a dangling folio pointer. >>> so using the folio_pfn() before dropping the reference, closing >>> use-after-free window. >>> >>> Fixes: d41fd2016ed0 ("mm/khugepaged: add tracepoint to hpage_collapse_scan_file()") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Vernon Yang >>> --- >>> include/trace/events/huge_memory.h | 6 +++--- >>> mm/khugepaged.c | 5 ++++- >>> 2 files changed, 7 insertions(+), 4 deletions(-) >>> >>> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h >>> index 5a48c5406cce..7b526528f85b 100644 >>> --- a/include/trace/events/huge_memory.h >>> +++ b/include/trace/events/huge_memory.h >>> @@ -178,10 +178,10 @@ TRACE_EVENT(mm_collapse_huge_page_swapin, >>> >>> TRACE_EVENT(mm_khugepaged_scan_file, >>> >>> - TP_PROTO(struct mm_struct *mm, struct folio *folio, struct file *file, >>> + TP_PROTO(struct mm_struct *mm, unsigned long pfn, struct file *file, >>> int present, int swap, int result), >>> >>> - TP_ARGS(mm, folio, file, present, swap, result), >>> + TP_ARGS(mm, pfn, file, present, swap, result), >>> >>> TP_STRUCT__entry( >>> __field(struct mm_struct *, mm) >>> @@ -194,7 +194,7 @@ TRACE_EVENT(mm_khugepaged_scan_file, >>> >>> TP_fast_assign( >>> __entry->mm = mm; >>> - __entry->pfn = folio ? folio_pfn(folio) : -1; >>> + __entry->pfn = pfn; >>> __assign_str(filename); >>> __entry->present = present; >>> __entry->swap = swap; >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 79effd3f3da4..00337405c0e0 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -2689,6 +2689,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, >>> int present, swap; >>> int node = NUMA_NO_NODE; >>> enum scan_result result = SCAN_SUCCEED; >>> + unsigned long pfn; >>> >>> present = 0; >>> swap = 0; >>> @@ -2719,6 +2720,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, >>> continue; >>> } >>> >>> + pfn = folio_pfn(folio); >>> if (is_pmd_order(folio_order(folio))) { >>> result = SCAN_PTE_MAPPED_HUGEPAGE; >>> /* >>> @@ -2779,7 +2781,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, >>> } >>> } >>> >>> - trace_mm_khugepaged_scan_file(mm, folio, file, present, swap, result); >>> + trace_mm_khugepaged_scan_file(mm, (!folio || xa_is_value(folio)) ? -1 : pfn, >>> + file, present, swap, result); >>> return result; >>> } >>> >> >> Shouldn't we just reset PFN to -1 at the beginning of the loop (and set it >> initially)? > > When the `xas_for_each()` iteration to terminate and the folio operation > preceding is normal, but pfn will be incorrect. The PFN is only relevant when a folio participated in the failure. Maybe the following would be cleanest? diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 75639298efc27..371ee0b16d10c 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2683,6 +2683,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, int present, swap; int node = NUMA_NO_NODE; enum scan_result result = SCAN_SUCCEED; + unsigned long problematic_pfn = -1; present = 0; swap = 0; @@ -2714,6 +2715,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, } if (is_pmd_order(folio_order(folio))) { + problematic_pfn = folio_pfn(folio); result = SCAN_PTE_MAPPED_HUGEPAGE; /* * PMD-sized THP implies that we can only try @@ -2725,6 +2727,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, node = folio_nid(folio); if (collapse_scan_abort(node, cc)) { + problematic_pfn = folio_pfn(folio); result = SCAN_SCAN_ABORT; folio_put(folio); break; @@ -2732,12 +2735,14 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, cc->node_load[node]++; if (!folio_test_lru(folio)) { + problematic_pfn = folio_pfn(folio); result = SCAN_PAGE_LRU; folio_put(folio); break; } if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) { + problematic_pfn = folio_pfn(folio); result = SCAN_PAGE_COUNT; folio_put(folio); break; @@ -2773,7 +2778,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, } } - trace_mm_khugepaged_scan_file(mm, folio, file, present, swap, result); + trace_mm_khugepaged_scan_file(mm, problematic_pfn, file, present, swap, result); return result; } -- Cheers, David