Re: [PATCH] mm: khugepaged: don't pass swap entry value to trace_mm_khugepaged_scan_file()

[Andrew Morton]

On Tue, 11 Aug 2026 21:36:55 +0800 Vernon Yang <vernon2gm@xxxxxxxxx> 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?

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@xxxxxxxxx


Also a possible bug mapping large folios which straddle i_size, which
is a separate thing.