Re: [PATCH v2] mm: khugepaged: fix memory leak in collapse_file xas retry loop

[Matthew Wilcox <willy@infradead.org>]

On Mon, Nov 24, 2025 at 09:41:49PM +0530, Shardul Bankar wrote:
> collapse_file() uses xas_create_range() in a retry loop that calls
> xas_nomem() on -ENOMEM and then retries. xas_nomem() may allocate a
> spare xa_node and store it in xas->xa_alloc.
> 
> If the lock is dropped after xas_nomem(), another thread can expand
> the xarray tree in the meantime. On the next retry, xas_create_range()
> can then succeed trivially without consuming the node stored in
> xas->xa_alloc. If we then either succeed or give up and go to the
> rollback path without calling xas_destroy(), that spare node leaks.

Then wouldn't freeing the excess node in xas_create_range() be the
correct fix, instead of requiring the caller to think about this?

> Fix this by calling xas_destroy(&xas) in both the success case
> (!xas_error(&xas)) and the failure case where xas_nomem() returns
> false and we abort. xas_destroy() will free any unused spare node in
> xas->xa_alloc and is a no-op if there is nothing left to free.
> 
> Link: https://syzkaller.appspot.com/bug?id=a274d65fc733448ed518ad15481ed575669dd98c
> Fixes: cae106dd67b9 ("mm/khugepaged: refactor collapse_file control flow")
> Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
> ---
>  v2:
>  - Call xas_destroy() on both success and failure
>  - Explained retry semantics and xa_alloc / concurrency risk
>  - Dropped cleanup_empty_nodes from previous proposal
> 
>  mm/khugepaged.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index abe54f0043c7..0794a99c807f 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1872,11 +1872,14 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>  	do {
>  		xas_lock_irq(&xas);
>  		xas_create_range(&xas);
> -		if (!xas_error(&xas))
> +		if (!xas_error(&xas)) {
> +			xas_destroy(&xas);
>  			break;
> +		}
>  		xas_unlock_irq(&xas);
>  		if (!xas_nomem(&xas, GFP_KERNEL)) {
>  			result = SCAN_FAIL;
> +			xas_destroy(&xas);
>  			goto rollback;
>  		}
>  	} while (1);
> -- 
> 2.34.1
> 
>