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Tue, 25 Aug 2026 19:47:09 -0700 (PDT) Date: Wed, 26 Aug 2026 10:47:03 +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 3/3] mm: khugepaged: fix folio is used after folio_put/unlock() Message-ID: <50049af3-ff42-4017-a535-aca124620087@gmail.com> References: <20260824092935.73892-1-vernon2gm@gmail.com> <20260824092935.73892-4-vernon2gm@gmail.com> <8001afe8-84e4-43d4-971b-b23f72070339@kernel.org> Precedence: bulk X-Mailing-List: stable@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: <8001afe8-84e4-43d4-971b-b23f72070339@kernel.org> On Mon, Aug 24, 2026 at 01:59:53PM +0200, David Hildenbrand (Arm) wrote: > On 8/24/26 11:29, Vernon Yang wrote: > > From: Vernon Yang > > > > On the rollback path, folio_put() has already dropped the last reference > > of new_folio. On the success path, new_folio is already unlocked and can > > be freed concurrently. The trace_mm_khugepaged_collapse_file() is left > > with a dangling folio pointer. > > > > So using the folio_pfn() before dropping the reference, closing > > use-after-free window. > > > > Fixes: 4c9473e87e75 ("mm/khugepaged: add tracepoint to collapse_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 fa828967e1fb..5fb4d92cfd84 100644 > > --- a/include/trace/events/huge_memory.h > > +++ b/include/trace/events/huge_memory.h > > @@ -211,10 +211,10 @@ TRACE_EVENT(mm_khugepaged_scan_file, > > ); > > > > TRACE_EVENT(mm_khugepaged_collapse_file, > > - TP_PROTO(struct mm_struct *mm, struct folio *new_folio, pgoff_t index, > > + TP_PROTO(struct mm_struct *mm, unsigned long new_pfn, pgoff_t index, > > unsigned long addr, bool is_shmem, struct file *file, > > int nr, int result), > > - TP_ARGS(mm, new_folio, index, addr, is_shmem, file, nr, result), > > + TP_ARGS(mm, new_pfn, index, addr, is_shmem, file, nr, result), > > TP_STRUCT__entry( > > __field(struct mm_struct *, mm) > > __field(unsigned long, hpfn) > > @@ -228,7 +228,7 @@ TRACE_EVENT(mm_khugepaged_collapse_file, > > > > TP_fast_assign( > > __entry->mm = mm; > > - __entry->hpfn = new_folio ? folio_pfn(new_folio) : -1; > > + __entry->hpfn = new_pfn; > > __entry->index = index; > > __entry->addr = addr; > > __entry->is_shmem = is_shmem; > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index 4e0fca5942dd..24347f1a94ae 100644 > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -2254,6 +2254,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, > > struct address_space *mapping = file->f_mapping; > > struct page *dst; > > struct folio *folio, *tmp, *new_folio; > > + unsigned long new_pfn = -1; > > pgoff_t index = 0, end = start + HPAGE_PMD_NR; > > LIST_HEAD(pagelist); > > XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER); > > @@ -2633,6 +2634,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, > > retract_page_tables(mapping, start); > > if (cc && !cc->is_khugepaged) > > result = SCAN_PTE_MAPPED_HUGEPAGE; > > + new_pfn = folio_pfn(new_folio); > > Why not set new_pfn once after successful alloc_charge_folio()? LGTM, I'll do it in the next version. Thanks! -- Cheers, Vernon