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Tue, 25 Aug 2026 19:46:16 -0700 (PDT) Date: Wed, 26 Aug 2026 10:46:11 +0800 From: Vernon Yang To: "David Hildenbrand (Arm)" 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 Subject: Re: [PATCH v3 2/3] mm: khugepaged: fix folio is used after pte_unmap_unlock() Message-ID: References: <20260824092935.73892-1-vernon2gm@gmail.com> <20260824092935.73892-3-vernon2gm@gmail.com> <0bc399df-2555-4378-bee4-eacbff434bdd@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0bc399df-2555-4378-bee4-eacbff434bdd@kernel.org> On Mon, Aug 24, 2026 at 01:57:15PM +0200, David Hildenbrand (Arm) wrote: > On 8/24/26 11:29, Vernon Yang wrote: > > From: Vernon Yang > > > > After the page table lock has dropped, the folio can be freed > > concurrently. The trace_mm_khugepaged_scan_pmd() is left with > > a dangling folio pointer. > > > > So using the folio_pfn() before dropping the page table lock, > > closing use-after-free window. > > > > Fixes: 7d2eba0557c1 ("mm: add tracepoint for scanning pages") > > 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 7b526528f85b..fa828967e1fb 100644 > > --- a/include/trace/events/huge_memory.h > > +++ b/include/trace/events/huge_memory.h > > @@ -55,10 +55,10 @@ SCAN_STATUS > > > > TRACE_EVENT(mm_khugepaged_scan_pmd, > > > > - TP_PROTO(struct mm_struct *mm, struct folio *folio, > > + TP_PROTO(struct mm_struct *mm, unsigned long pfn, > > int referenced, int none_or_zero, int status, int unmapped), > > > > - TP_ARGS(mm, folio, referenced, none_or_zero, status, unmapped), > > + TP_ARGS(mm, pfn, referenced, none_or_zero, status, unmapped), > > > > TP_STRUCT__entry( > > __field(struct mm_struct *, mm) > > @@ -71,7 +71,7 @@ TRACE_EVENT(mm_khugepaged_scan_pmd, > > > > TP_fast_assign( > > __entry->mm = mm; > > - __entry->pfn = folio ? folio_pfn(folio) : -1; > > + __entry->pfn = pfn; > > __entry->referenced = referenced; > > __entry->none_or_zero = none_or_zero; > > __entry->status = status; > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index 00337405c0e0..4e0fca5942dd 100644 > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -1618,6 +1618,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, > > enum scan_result result = SCAN_FAIL; > > struct page *page = NULL; > > struct folio *folio = NULL; > > + unsigned long pfn = -1; > > unsigned long addr; > > unsigned long enabled_orders; > > spinlock_t *ptl; > > @@ -1780,6 +1781,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, > > result = SCAN_SUCCEED; > > } > > out_unmap: > > + if (folio) > > + pfn = folio_pfn(folio); > > Should we reset the folio to NULL at the beginning of the loop? Then we really > only trace the PFN if it really was problematic. Yes, this is a pre-existing bug, and I'll fix it together. Thanks! But it is not at the beginning of the loop, it is at the __ending__ of the loop, for the same reason as PATCH#1. -- Cheers, Vernon