Re: [PATCH 2/2] mm/page_owner: also check PageTail before reading memcg_data

[Zi Yan]

On Wed Nov 4, 2026 at 4:59 AM EST, Daejun Park via B4 Relay wrote:
> From: Daejun Park <daejun7.park@xxxxxxxxxxx>
>
> print_page_owner_memcg() loads memcg_data before it tests PageTail().
> In the first tail page, memcg_data holds the folio's _nr_pages. A split
> can make the page a folio between the two loads. The stale _nr_pages
> then passes the PageTail() test and is used as an objcg pointer.
>
> __split_folio_to_order() stores memcg_data before the smp_wmb() that
> precedes clear_compound_head(). Also test PageTail() before the load,
> with a paired smp_rmb(). Keep the test after the load for a page that
> becomes a tail in between.
>
> Fixes: 4996fc547f5b ("mm: let _folio_nr_pages overlay memcg_data in first tail page")
> Signed-off-by: Daejun Park <daejun7.park@xxxxxxxxxxx>
> ---
> Needs patch 1 for the store order. Found by reading the code, not
> reproduced. The page_owner oops seen with a delay after
> clear_compound_head() is the window patch 1 closes.
> ---
>  mm/page_owner.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>

Makes sense.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

-- 
Best Regards,
Yan, Zi