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Tue, 25 Aug 2026 19:44:51 -0700 (PDT) Date: Wed, 26 Aug 2026 10:44:45 +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 1/3] mm: khugepaged: fix swap entry value to folio_pfn() Message-ID: <6a9c2369-5589-4f2a-bcfe-c6e3b46a1ccd@gmail.com> References: <20260824092935.73892-1-vernon2gm@gmail.com> <20260824092935.73892-2-vernon2gm@gmail.com> 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: 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. If we just reset PFN to -1 at the __ending__ of the loop (and set it initially), then the above situation won't occur. If so, I will do it in the next version. -- Cheers, Vernon