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Wed, 12 Aug 2026 07:08:57 -0700 (PDT) Date: Wed, 12 Aug 2026 22:08:49 +0800 From: Vernon Yang To: Andrew Morton Cc: david@kernel.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, Vernon Yang , stable@vger.kernel.org Subject: Re: [PATCH] mm: khugepaged: don't pass swap entry value to trace_mm_khugepaged_scan_file() Message-ID: <7328756b-ad4f-4cfe-8d55-4659510212de@gmail.com> References: <20260811133655.267739-1-vernon2gm@gmail.com> <20260811121225.fe45676262404b82270843a1@linux-foundation.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: <20260811121225.fe45676262404b82270843a1@linux-foundation.org> On Tue, Aug 11, 2026 at 12:12:25PM -0700, Andrew Morton wrote: > On Tue, 11 Aug 2026 21:36:55 +0800 Vernon Yang wrote: > > > 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 set folio to NULL before breaking out, the tracepoint maps NULL to > > scan_pfn of -1, just like exhausted scan naturally. > > > > Fixes: d41fd2016ed0 ("mm/khugepaged: add tracepoint to hpage_collapse_scan_file()") > > Added in 2022. Why so long - do people not use tracing? For most architectures, SPARSEMEM_VMEMMAP is the default, where page_to_pfn() is plain pointer arithmetic, merely printing bogus scan_pfn. Only on classic SPARSEMEM, the swap entry value cause khugepaged to oops. > Sashiko might have a found a couple of other tracing bugs in this code, > which I suggest are on-topic for your patch: > > https://sashiko.dev/#/patchset/20260811133655.267739-1-vernon2gm@gmail.com > David also mentioned "Using the folio after dropping the reference", and the same issue exists in trace_mm_khugepaged_collapse_file()/trace_mm_khugepaged_scan_pmd(), which merely prints a overdue scan_pfn without oopsing khugepaged. However, this approach does have potential issues, just that they haven't been triggered yet. I can fix them together. I will add PATCH#2 and PATCH#3 in the next version of the patchset to fix the other two trace_xxx() functions, rather than mixing them together into one large patch. > Also a possible bug mapping large folios which straddle i_size, which > is a separate thing. Yes, I will submit a separate fix patch later to resolve this issue. -- Cheers, Vernon