[PATCH 01/46] mm/mglru: don't use stack pointers when walking PUD and PMD tables

[Barry Song]

On Tue, Oct 27, 2026 at 7:18 AM David Hildenbrand (Arm)
<david at kernel.org> wrote:
>
> From: Josef Bacik <josef at toxicpanda.com>
>
> MGLRU passes stack copies of page-table entries in walk_pud_range() and
> walk_pmd_range() when walking to the next level. These copies can become
> stale when khugepaged or MADV_DONTNEED (CONFIG_PT_RECLAIM) removes a PTE
> table concurrently.
>
> This pattern dates back to commit bd74fdaea146 ("mm: multi-gen LRU:
> support page table walks"). Likely, earlier locking masked the latent
> bug. Later changes exposed it: commit 1d65b771bc08 ("mm/khugepaged:
> retract_page_tables() without mmap or vma lock") allowed concurrent
> retraction, and commit cf4a65539c13 ("memcg-v1: no need for memcg locking
> for MGLRU") dropped the walker's outer RCU protection.
>
> The fix is simple, so just fix the original commit so we don't have to
> speculate. To fix it, pass the actual PMD/PUD pointers. This restores the
> RCU-protected lookup and validation against the live PMD. Note that the PUD
> handling is required as well to handle folded page tables correctly in all
> cases.
>
> Fix generated by an LLM and sent by Josef Bacik in private.
>
> Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
> Cc: stable at vger.kernel.org
> Signed-off-by: Josef Bacik <josef at toxicpanda.com>
> [ david: handle walk_pud_range() as well, clarify+simplify description,
>          adjust Fixes: ]
> Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
> ---

Reviewed-by: Barry Song <baohua at kernel.org>