maple_tree: fix MA_STATE_PREALLOC flag in mas_preallocate()

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)

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 54a611b ("Maple Tree: add new data structure")
Signed-off-by: Liam R. Howlett <[email protected]>
Reported-by: Zhaoyang Huang <[email protected]>
Reported-by: Hailong Liu <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Link: https://lore.kernel.org/all/[email protected]/
Link: https://lore.kernel.org/all/[email protected]/
Cc: Lorenzo Stoakes <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Hailong Liu <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Steve Kang <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: howlett

lib/maple_tree.c

@@ -5527,8 +5527,9 @@ 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);
@@ -5538,6 +5539,7 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
 		return ret;
 	}
 
+set_flag:
 	mas->mas_flags |= MA_STATE_PREALLOC;
 	return ret;
 }