Re: [PATCH 5.15.y 6.1.y 6.6.y 1/1] mm/vmscan: flush deferred TLB before freeing large folios

[Matthew Wilcox]

On Wed, Jul 08, 2026 at 12:12:36PM +0800, Jiayuan Chen wrote:
> In reclaim, shrink_folio_list() unmaps PTEs with a deferred, batched TLB
> flush. The batch is only flushed by try_to_unmap_flush() near the end of
> the function, just before the order-0 folios collected in @free_folios are
> handed back to the allocator.
> 
> Large folios don't go through @free_folios -- they're freed inline at the
> free_it label via destroy_large_folio(), which runs before that flush. So
> a large folio's pages can be returned to the buddy allocator and reused
> while another CPU still holds a stale TLB entry for them, and that CPU then
> reads or executes through the stale translation into the reused page. For
> file-backed large folios (e.g. executable text) this shows up as random
> SIGSEGV/SIGILL in user space, with fault addresses that don't match the
> code being run.
> 
> Flush the deferred batch before freeing a large folio inline, the same way
> the order-0 path already waits for the flush.
> 
> Upstream this is fixed as a side effect of commit bc2ff4cbc329 ("mm: free
> folios in a batch in shrink_folio_list()"), which is a larger change; this
> is the minimal fix for -stable.

I agree that it would be wrong to backport bc2ff4cbc329 and all its
dependencies.  And free_unref_page_list() only handles order-0 folios,
so we can't do this:

-		if (unlikely(folio_test_large(folio)))
-			destroy_large_folio(folio);
-		else
-			list_add(&folio->lru, &free_folios);
+		list_add(&folio->lru, &free_folios);
 		continue;

I think this is the right fix.

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

> Reported-by: Yingfu Zhou <yingfu.zhou@xxxxxxxxxx>
> Fixes: bd4c82c22c36 ("mm, THP, swap: delay splitting THP after swapped out")
> Cc: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
> ---
> destroy_compound_page was recently renamed to destroy_large_folio.
> So it would be conflict when this patch was applied to 5.15/6.1
> ---
>  mm/vmscan.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index aba757e5c597..8eb498351d9b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2123,10 +2123,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  		 * Is there need to periodically free_folio_list? It would
>  		 * appear not as the counts should be low
>  		 */
> -		if (unlikely(folio_test_large(folio)))
> +		if (unlikely(folio_test_large(folio))) {
> +			try_to_unmap_flush();
>  			destroy_large_folio(folio);
> -		else
> +		} else {
>  			list_add(&folio->lru, &free_folios);
> +		}
>  		continue;
>  
>  activate_locked_split:
> -- 
> 2.43.0
>