Re: [PATCH v4] lib: xarray: free unused spare node in xas_create_range()
From: Jinjiang Tu
Date: Sun Dec 14 2025 - 22:42:29 EST
在 2025/12/15 10:19, Jinjiang Tu 写道:
在 2025/12/4 22:26, Shardul Bankar 写道:
xas_create_range() is typically called in a retry loop that usesI encountered another memory leak issue in xas_create_range().
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().
collapse_file() calls xas_create_range() to pre-create all slots needed.
If collapse_file() finally fails, these pre-created slots are empty nodes.
When the file is deleted, shmem_evict_inode()->shmem_truncate_range()->shmem_undo_range()
calls xas_store(&xas, NULL) for each entries to delete nodes, but leaving those pre-created
empty nodes leaked.
I can reproduce it with following steps.
1) create file /tmp/test_madvise_collapse and ftruncate to 4MB size, and then mmap the file
2) memset for the first 2MB
3) madvise(MADV_COLLAPSE) for the second 2MB.
4) unlink the file
in 3), collapse_file() calls xas_create_range() to expand xarray depth, and fails to collapse
due to the whole 2M region is empty, leading to the new created empty nodes leaked.
To fix it, maybe we should add a new function xas_delete_range() to revert what xas_create_range()
does when xas_create_range() runs into rollback path?
How about the following diff? I tried it, and the memory leak disappears. I'm new in xarray, so
I don't if this fix works properly.
diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index be850174e802..972df5ceeb84 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -1555,6 +1555,7 @@ void xas_destroy(struct xa_state *);
void xas_pause(struct xa_state *);
void xas_create_range(struct xa_state *);
+void xas_destroy_range(struct xa_state *xas, unsigned long start, unsigned long end);
#ifdef CONFIG_XARRAY_MULTI
int xa_get_order(struct xarray *, unsigned long index);
diff --git a/lib/xarray.c b/lib/xarray.c
index 9a8b4916540c..ab15dc939962 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -752,6 +752,21 @@ void xas_create_range(struct xa_state *xas)
}
EXPORT_SYMBOL_GPL(xas_create_range);
+void xas_destroy_range(struct xa_state *xas, unsigned long start, unsigned long end)
+{
+ unsigned long index;
+ void *entry;
+
+ for (index = start; index < end; ++index) {
+ xas_set(xas, index);
+ entry = xas_load(xas);
+ if (entry)
+ continue;
+ else if (xas->xa_node && !xas->xa_node->count)
+ xas_delete_node(xas);
+ }
+}
+
static void update_node(struct xa_state *xas, struct xa_node *node,
int count, int values)
{
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 97d1b2824386..dd9d3f202c4b 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2247,7 +2247,10 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
goto out;
rollback:
- /* Something went wrong: roll back page cache changes */
+ /* Something went wrong: roll back empty xa_node created by
+ * xas_create_range() and page cache changes
+ */
+ xas_destroy_range(&xas, start, end);
+
if (nr_none) {
xas_lock_irq(&xas);
mapping->nrpages -= nr_none;
Link:https://syzkaller.appspot.com/bug?id=a274d65fc733448ed518ad15481ed575669dd98c
Link:https://lore.kernel.org/all/20251201074540.3576327-1-shardul.b@xxxxxxxxxxxxxxxxxx/ ("v3")
Fixes: cae106dd67b9 ("mm/khugepaged: refactor collapse_file control flow")
Signed-off-by: Shardul Bankar<shardul.b@xxxxxxxxxxxxxxxxxx>
---
v4:
- Drop redundant `if (xa_alloc)` around xas_destroy(), as xas_destroy()
already checks xa_alloc internally.
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 | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/lib/xarray.c b/lib/xarray.c
index 9a8b4916540c..f49ccfa5f57d 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -744,11 +744,16 @@ 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() */
+ xas_destroy(xas);
}
EXPORT_SYMBOL_GPL(xas_create_range);