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Mon, 07 Sep 2026 18:35:40 -0700 (PDT) Date: Tue, 8 Sep 2026 09:35:33 +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 v4 1/3] mm: khugepaged: fix swap entry value to folio_pfn() Message-ID: <047c287f-bb00-4672-9ab8-3323cbae8f2e@gmail.com> References: <20260828055926.346744-1-vernon2gm@gmail.com> <20260828055926.346744-2-vernon2gm@gmail.com> <53f3afde-b5a7-4c2e-b6b5-8c844d1b6c8e@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: <53f3afde-b5a7-4c2e-b6b5-8c844d1b6c8e@kernel.org> On Mon, Sep 07, 2026 at 02:25:57PM +0200, David Hildenbrand (Arm) wrote: > On 8/28/26 07:59, 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. > > > > About calling the respective trace_xxx() functions separately on success > > and failure, refer to [1]. > > > > [1] https://lore.kernel.org/linux-mm/ao6jVbVHLUmuY2UA@gremlin/ > > > > 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 | 11 ++++++++++- > > 2 files changed, 13 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 75639298efc2..b597a3e68606 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 failed_pfn = -1; > > > > present = 0; > > swap = 0; > > @@ -2715,6 +2716,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, > > > > if (is_pmd_order(folio_order(folio))) { > > result = SCAN_PTE_MAPPED_HUGEPAGE; > > + failed_pfn = folio_pfn(folio); > > /* > > * PMD-sized THP implies that we can only try > > * retracting the PTE table. > > @@ -2726,6 +2728,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, > > node = folio_nid(folio); > > if (collapse_scan_abort(node, cc)) { > > result = SCAN_SCAN_ABORT; > > + failed_pfn = folio_pfn(folio); > > folio_put(folio); > > break; > > } > > @@ -2733,12 +2736,14 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, > > > > if (!folio_test_lru(folio)) { > > result = SCAN_PAGE_LRU; > > + failed_pfn = folio_pfn(folio); > > folio_put(folio); > > break; > > } > > > > if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) { > > result = SCAN_PAGE_COUNT; > > + failed_pfn = folio_pfn(folio); > > folio_put(folio); > > break; > > } > > @@ -2771,9 +2776,13 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, > > } else { > > result = collapse_file(mm, addr, file, start, cc); > > } > > + trace_mm_khugepaged_scan_file(mm, -1, file, present, swap, > > + SCAN_SUCCEED); > > + } else { > > + trace_mm_khugepaged_scan_file(mm, failed_pfn, file, present, > > + swap, result); > > } > > Just curious (maybe Lorenzo suggested that?), why separate out the SCAN_SUCCEED > stuff? Yes, this is the approach Lorenzo[1] suggested in the previous version. [1] https://lore.kernel.org/linux-mm/ao6jVbVHLUmuY2UA@gremlin/ > When result == SCAN_SUCCEED, failed_pfn == -1 no? Your understanding is completely correct, and I explained at the time that this was redundant. > We set failed_pfn now only when setting result ... If you prefer one trace_xxx(), please let me know, and I'd be happy to send V5. -- Cheers, Vernon