[PATCH 2/3] maple_tree: Fix MA_STATE_PREALLOC flag in mas_preallocate()

[Liam R. Howlett]

Temporarily clear the preallocation flag when explicitly requesting
allocations.  Pre-existing allocations are already counted against the
request through mas_node_count_gfp(), but the allocations will not
happen if the MA_STATE_PREALLOC flag is set.  This flag is meant to
avoid re-allocating in bulk allocation mode, and to detect issues with
preallocation calculations.

The MA_STATE_PREALLOC flag should also always be set on zero allocations
so that detection of underflow allocations will print a WARN_ON() during
consumption.

User visible effect of this flaw is a WARN_ON() followed by a null
pointer dereference when subsequent requests for larger number of nodes
is ignored, such as the vma merge retry in mmap_region() caused by
drivers altering the vma flags (which happens in v6.6, at least)

Reported-by: Zhaoyang Huang <zhaoyang.huang at unisoc.com>
Reported-by: Hailong Liu <hailong.liu at oppo.com>
Fixes: 54a611b605901 ("Maple Tree: add new data structure")
Link: https://lore.kernel.org/all/1652f7eb-a51b-4fee-8058-c73af63bacd1@oppo.com/
Link: https://lore.kernel.org/all/20250428184058.1416274-1-Liam.Howlett@oracle.com/
Link: https://lore.kernel.org/all/20250429014754.1479118-1-Liam.Howlett@oracle.com/
Cc: stable at kernel.org
Cc: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
Cc: Suren Baghdasaryan <surenb at google.com>
Cc: Hailong Liu <hailong.liu at oppo.com>
Cc: zhangpeng.00 at bytedance.com <zhangpeng.00 at bytedance.com>
Cc: Steve Kang <Steve.Kang at unisoc.com>
Cc: Matthew Wilcox <willy at infradead.org>
Cc: Sidhartha Kumar <sidhartha.kumar at oracle.com>
Signed-off-by: Liam R. Howlett <Liam.Howlett at oracle.com>
---
 lib/maple_tree.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 7144dbbc34813..54a09c26edf96 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5528,8 +5528,9 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
 	mas->store_type = mas_wr_store_type(&wr_mas);
 	request = mas_prealloc_calc(&wr_mas, entry);
 	if (!request)
-		return ret;
+		goto set_flag;
 
+	mas->mas_flags &= ~MA_STATE_PREALLOC;
 	mas_node_count_gfp(mas, request, gfp);
 	if (mas_is_err(mas)) {
 		mas_set_alloc_req(mas, 0);
@@ -5539,6 +5540,7 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
 		return ret;
 	}
 
+set_flag:
 	mas->mas_flags |= MA_STATE_PREALLOC;
 	return ret;
 }
-- 
2.47.2