Re: [PATCH v3] lib: xarray: free unused spare node in xas_create_range()

[David Hildenbrand (Red Hat)]

On 12/1/25 08:45, Shardul Bankar wrote:

Please don't post new versions as reply to old versions.

> xas_create_range() is typically called in a retry loop that uses
> xas_nomem() to handle -ENOMEM errors. xas_nomem() may allocate a spare
> xa_node and store it in xas->xa_alloc for use in the retry.
>
> 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 without consuming the spare node stored in xas->xa_alloc.
> If the function returns without freeing this spare node, it leaks.
>
> xas_create_range() calls xas_create() multiple times in a loop for
> different index ranges. A spare node that isn't needed for one range
> iteration might be needed for the next, so we cannot free it after each
> xas_create() call. We can only safely free it after xas_create_range()
> completes.
>
> Fix this by calling xas_destroy() at the end of xas_create_range() to
> free any unused spare node. This makes the API safer by default and
> prevents callers from needing to remember cleanup.
>
> This fixes a memory leak in mm/khugepaged.c and potentially other
> callers that use xas_nomem() with xas_create_range().
>
> Link: https://syzkaller.appspot.com/bug?id=a274d65fc733448ed518ad15481ed575669dd98c
> Fixes: cae106dd67b9 ("mm/khugepaged: refactor collapse_file control flow")
> Signed-off-by: Shardul Bankar <shardul.b@xxxxxxxxxxxxxxxxxx>
> ---
>   v3:
>   - Move fix from collapse_file() to xas_create_range() as suggested by Matthew Wilcox
>   - Fix in library function makes API safer by default, preventing callers from needing
>     to remember cleanup
>   - Use shared cleanup label that both restore: and success: paths jump to
>   - Clean up unused spare node on both success and error exit paths
>   v2:
>   - Call xas_destroy() on both success and failure
>   - Explained retry semantics and xa_alloc / concurrency risk
>   - Dropped cleanup_empty_nodes from previous proposal
>   lib/xarray.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/lib/xarray.c b/lib/xarray.c
> index 9a8b4916540c..a924421c0c4c 100644
> --- a/lib/xarray.c
> +++ b/lib/xarray.c
> @@ -744,11 +744,17 @@ void xas_create_range(struct xa_state *xas)
>   	xas->xa_shift = shift;
>   	xas->xa_sibs = sibs;
>   	xas->xa_index = index;
> -	return;
> +	goto cleanup;
> +
>   success:
>   	xas->xa_index = index;
>   	if (xas->xa_node)
>   		xas_set_offset(xas);
> +
> +cleanup:
> +	/* Free any unused spare node from xas_nomem() */
> +	if (xas->xa_alloc)
> +		xas_destroy(xas);

The first thing xas_destroy() does is check whether xa_alloc is set.

I'd assume that the compiler is smart enough to inline xas_destroy() 
completely here, so likely the xa_alloc check here can just be dropped.


Staring at xas_destroy() callers, we only have a single one outside of 
lib: mm/huge_memory.c:__folio_split()

Is that one still required?

--
Cheers

David