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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260811121225.fe45676262404b82270843a1@linux-foundation.org> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 8B8ED40012 X-Stat-Signature: 3sjggkxkfbrg73dusef7z4i9gqnqwgh9 X-Rspam-User: X-HE-Tag: 1786543739-834800 X-HE-Meta: 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 k0BsOoel 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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