[PATCH 14/18] maple_tree: WARN_ON_ONCE when allocations fail

["Liam R. Howlett (Oracle)" <liam@infradead.org>]

Allocations should never fail in the circumstances that are expected to
occur.  Add checks in the code to ensure the circumstances are correctly
set up by the user and warn if they are not.

Also add a warning on failure to allocate, which should never happen.

Cc: Rik van Riel <riel@surriel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Liam R. Howlett (Oracle) <liam@infradead.org>
---
 lib/maple_tree.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index c5efa3e2f7461..94f2d5b9bc881 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5747,6 +5747,10 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
 	if (likely(mas->node != MA_ERROR(-ENOMEM)))
 		return false;
 
+	/* Allocations can fail, don't do this. */
+	WARN_ON_ONCE(!gfpflags_allow_blocking(gfp) &&
+		     mt_external_lock(mas->tree));
+
 	if (gfpflags_allow_blocking(gfp) && !mt_external_lock(mas->tree)) {
 		mtree_unlock(mas->tree);
 		mas_alloc_nodes(mas, gfp);
@@ -5757,9 +5761,12 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
 
 	/*
 	 * Return false on zero forward progress.  Partial allocations are kept
-	 * so the retry path will attempt to get the rest.
+	 * so the retry path will attempt to get the rest.  The failure should
+	 * not happen as we try our best to reclaim.  The user would need an
+	 * external lock with a non-blocking gfp in a low memory situation -
+	 * which would have triggered the first warning in this function.
 	 */
-	if (!mas->sheaf && !mas->alloc)
+	if (WARN_ON_ONCE(!mas->sheaf && !mas->alloc))
 		return false;
 
 	mas_reset(mas);
-- 
2.47.3