Re: [PATCH] mm/swap_cgroup: fix kernel BUG in swap_cgroup_record

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 10 Jan 2026 12:16:13 +0530 Deepanshu Kartikey <kartikey406@gmail.com> wrote:

> When using MADV_PAGEOUT, pages can remain in swapcache with their swap
> entries assigned. If MADV_PAGEOUT is called again on these pages, they
> reuse the same swap entries, causing memcg1_swapout() to call
> swap_cgroup_record() with an already-recorded entry.
> 
> The existing code assumes swap entries are always being recorded for the
> first time (oldid == 0), triggering VM_BUG_ON when it encounters an
> already-recorded entry:
> 
>   ------------[ cut here ]------------
>   kernel BUG at mm/swap_cgroup.c:78!
>   Oops: invalid opcode: 0000 [#1] SMP KASAN PTI
>   CPU: 0 UID: 0 PID: 6176 Comm: syz.0.30 Not tainted
>   RIP: 0010:swap_cgroup_record+0x19c/0x1c0 mm/swap_cgroup.c:78
>   Call Trace:
>    memcg1_swapout+0x2fa/0x830 mm/memcontrol-v1.c:623
>    __remove_mapping+0xac5/0xe30 mm/vmscan.c:773
>    shrink_folio_list+0x2786/0x4f40 mm/vmscan.c:1528
>    reclaim_folio_list+0xeb/0x4e0 mm/vmscan.c:2208
>    reclaim_pages+0x454/0x520 mm/vmscan.c:2245
>    madvise_cold_or_pageout_pte_range+0x19a0/0x1ce0 mm/madvise.c:563
>    ...
>    do_madvise+0x1bc/0x270 mm/madvise.c:2030
>    __do_sys_madvise mm/madvise.c:2039
> 
> This bug occurs because pages in swapcache can be targeted by
> MADV_PAGEOUT multiple times without being swapped in between. Each time,
> the same swap entry is reused, but swap_cgroup_record() expects to only
> record new, unused entries.
> 
> Fix this by checking if the swap entry already has the correct cgroup ID
> recorded before attempting to record it. Use the existing
> lookup_swap_cgroup_id() to read the current cgroup ID, and return early
> from memcg1_swapout() if the entry is already correctly recorded. Only
> call swap_cgroup_record() when the entry needs to be set or updated.
> 
> This approach avoids unnecessary atomic operations, reference count
> manipulations, and statistics updates when the entry is already correct.

Thanks.  This looks like a fairly old bug and it annoyingly predates a
lot of memcg code movement.

What do people think?  Should we backport this into -stable kernels? 
If so, can some intrepid soul please help figure out what it Fixes:?

Deepanshu, if we do decide to put a cc:stable on this then some -stable
maintainers will complain that the patch alters things in a file which
doesn't exist and they'll hope that you can help.  Which means
backporting the fix into kernels which predate 89ce924f0bd44.