From: Andrew Morton <akpm@linux-foundation.org>
To: Vernon Yang <vernon2gm@gmail.com>
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 <yanglincheng@kylinos.cn>,
stable@vger.kernel.org
Subject: Re: [PATCH] mm: khugepaged: don't pass swap entry value to trace_mm_khugepaged_scan_file()
Date: Tue, 11 Aug 2026 12:12:25 -0700 [thread overview]
Message-ID: <20260811121225.fe45676262404b82270843a1@linux-foundation.org> (raw)
In-Reply-To: <20260811133655.267739-1-vernon2gm@gmail.com>
On Tue, 11 Aug 2026 21:36:55 +0800 Vernon Yang <vernon2gm@gmail.com> 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@gmail.com
Also a possible bug mapping large folios which straddle i_size, which
is a separate thing.
next prev parent reply other threads:[~2026-08-11 19:12 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-11 13:36 [PATCH] mm: khugepaged: don't pass swap entry value to trace_mm_khugepaged_scan_file() Vernon Yang
2026-08-11 15:19 ` David Hildenbrand (Arm)
2026-08-12 14:03 ` Vernon Yang
2026-08-11 19:12 ` Andrew Morton [this message]
2026-08-12 14:08 ` Vernon Yang
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