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b=DO/5UK1ssNLcrIayHml0xY4L03vESqdwM2OervIq1HvNArbtmpckEwdqoiTlZEWmr axhCn3wOd9mP7m3iLZ4K8rP3Gev/yICjRZ9+/7QSY5g4v+Gg5CXOivz6reTfffXAx3 JuieUNZntzFC7sC5Vuz0P7OQUoeWe9ux67QlWReeD/lDwkpkULnkt8jJTNKRDErRDl KjxzSX9ogCtAoCRtNInsga6MOBDMYoNXIt0LM634XYTmVijSTwDh0LgzrTRJSs+XLY 4SUL2VQNv3gTx8UglzVgiIgvGdvuR8COvHauoqpBUDY0U1ZHYU3PDAafRSu2ue7F/9 CAp1wSHtrPCIQ== Date: Mon, 17 Aug 2026 17:23:30 +0100 From: "Lorenzo Stoakes (ARM)" To: Vernon Yang Cc: akpm@linux-foundation.org, david@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, Vernon Yang , stable@vger.kernel.org Subject: Re: [PATCH v2 1/3] mm: khugepaged: fix swap entry value to folio_pfn() Message-ID: References: <20260815051924.194810-1-vernon2gm@gmail.com> <20260815051924.194810-2-vernon2gm@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 25EE040005 X-Stat-Signature: ue8fshguae6zpo8ajybm1pqkpixncxxw X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1786983829-688046 X-HE-Meta: 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 oD0J1aSB DfgdNvnMitazwfAof3MeX3MvpCIft4ciC8mefL2cwY2U+2tSQ9GYvdcg35tOpU+qS8JYvm4V+F1yud0eM7bkJGm8+xkk6GLJoLmEi33Avb1pckTJtYFn73jGiXtuA+Y7g6eo2i09CT5dA4YS8TRZk2S5K5eWRW1iKRgBj6KbHKP4vj2G0umW8Str6IwFEEtjp/eMzJobAw8naHz0LjsTDLzUW9pMhVGHMUenncrwaRf3hCy7NaOFGzScU/FJUMWRErdKRPDaVnD6DXpoAph0q/Rlww1fMkoNkj1G5FwNHc0Wa8V0h7IGaPuAhieDXzQ/HPviD Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 17, 2026 at 05:10:39PM +0100, Lorenzo Stoakes (ARM) wrote: > On Sat, Aug 15, 2026 at 01:19:22PM +0800, 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. > > Hmm I seem to recall that we were no longer spporting !VMEMMAP SPARSEMEM, but > maybe I was imagining that :) OK seems some museum piece architectures are causing us issues again, fun times (parisc and some mips). > > But this needs fixing in general anyway. > > > > > 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 | 14 +++++++++----- > > 2 files changed, 12 insertions(+), 8 deletions(-) > > > > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h > > index 291fae364c62..d3572d4ef453 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; > > I wonder how easy it is for people to interpret that this is a PFN of something > only in the case of an early exit. > > It's a bit of a mess that we're exposing internal implementation details like > this, quite honestly. > > I hope there is no expectation of this being there indefinitely. > > > __assign_str(filename); > > __entry->present = present; > > __entry->swap = swap; > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index 617bca76db49..e7830761d3a2 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 pfn; > > See below I think we should drop it. OK we can't because of museum piece architectures that will actually deref the folio to get the PFN... I'd startr by initialising this to -1 then since I guess in theory it's possible xas_for_each() could just not do anything is it? (folio is being set to NULL so that suggests so). > > > > > present = 0; > > swap = 0; > > @@ -2720,27 +2721,23 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, > > * PMD-sized THP implies that we can only try > > * retracting the PTE table. > > */ > > - folio_put(folio); I still hate the idea of moving these outside of the loop, it's just asking for trouble. So I guess instead: xas_for_each(&xas, folio, start + HPAGE_PMD_NR - 1) { pfn = -1; ... if (xa_is_value(folio)) { ... } pfn = folio_pfn(folio); ... } ... trace_mm_khugepaged_scan_file(mm, pfn, file, present, swap, result); That way you avoid the horror or moving the folio put somewhere deeply unintuitive and inconsistent, and maintain pfn lifetime for some arches nobody uses. > > break; > > } > > > > node = folio_nid(folio); > > if (collapse_scan_abort(node, cc)) { > > result = SCAN_SCAN_ABORT; > > - folio_put(folio); > > break; > > } > > cc->node_load[node]++; > > > > if (!folio_test_lru(folio)) { > > result = SCAN_PAGE_LRU; > > - folio_put(folio); > > break; > > } > > > > if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) { > > result = SCAN_PAGE_COUNT; > > - folio_put(folio); > > break; > > } > > > > @@ -2759,7 +2756,14 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, > > cond_resched_rcu(); > > } > > } > > + if (!folio || xa_is_value(folio)) { > > + pfn = -1; > > + } else { > > + pfn = folio_pfn(folio); > > + folio_put(folio); > > + } > > Hmm kinda hate this you're now mixing clean up logic with some trace logic. > > You have to go check out xas_for_each() and see that xas_next_entry() sets folio > to NULL at the end for the normal case to know this is OK and then figure out > that this is just for the break cases. > > That's not intuitive in already not intuitive code. > > And this is a patch you're wanting to backport to fix a bug too... :) > > I think you should keep the folio_put()'s where they are for now and limit this > fix to the trace code please. > > I think you should drop the pfn var altogether and do this far more simply, see > below. > > > rcu_read_unlock(); > > + > > Stray newline added, let's not do that for a backported fix please :) > > > if (result == SCAN_PTE_MAPPED_HUGEPAGE) > > cc->progress++; > > else > > @@ -2774,7 +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, pfn, file, present, swap, result); > > I mean maybe better to just reduce this to something like: > > /* folio is NULL unless exited early. */ > trace_mm_khugepaged_scan_file(mm, > (!folio || xa_is_value(folio)) ? -1 : folio_pfn(folio), > file, present, swap, result); > > As the least invasive way of fixing the trace + limiting the contageon? Yeah as above this isn't right. > > > return result; > > } > > > > -- > > 2.53.0 > > > > -- > Cheers, Lorenzo -- Cheers, Lorenzo