[PATCH] mm/vmalloc: do not warn on -ENOMEM from va_clip() in pcpu_get_vm_areas()

[Palla Raghunath]

When pcpu_get_vm_areas() has to split a free vmap_area in the middle
(NE_FIT_TYPE), va_clip() needs an extra vmap_area object. It takes the
per-cpu ne_fit_preload_node if one is there, and otherwise falls back
to kmem_cache_alloc(GFP_NOWAIT), which may fail and return -ENOMEM.
pcpu_get_vm_areas() never preloads, and a single call can do more than
one such split: on a NUMA system it places one area per node group, so
the first split consumes the preloaded object and the next one depends
on the GFP_NOWAIT allocation.

That failure is expected and already handled: the recovery path returns
the areas clipped so far to the free tree, purges lazily freed areas and
retries. But the error is checked with WARN_ON_ONCE(), so a transient
allocation failure under memory pressure or fault injection triggers a
kernel warning, and a panic with panic_on_warn. syzbot hit this on a
two-node VM while creating a per-cpu BPF array map.

Keep the WARN_ON_ONCE() for errors other than -ENOMEM, which do indicate
a bug, and take the recovery path either way. This matches what commit
b9183788a2de ("mm/vmalloc: do not warn on -ENOMEM from va_alloc()") did
for the other va_clip() caller.

Fixes: 1b23ff80b399 ("mm/vmalloc: invoke classify_va_fit_type() in adjust_va_to_fit_type()")
Reported-by: syzbot+442828bb356b10813a47@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=442828bb356b10813a47
Signed-off-by: Palla Raghunath <raghunathpalla.0209@xxxxxxxxx>
---
 mm/vmalloc.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index bea9f76ed7e7..24c7d0a5472e 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -5107,9 +5107,14 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
 
 		ret = va_clip(&free_vmap_area_root,
 			&free_vmap_area_list, va, start, size);
-		if (WARN_ON_ONCE(unlikely(ret)))
-			/* It is a BUG(), but trigger recovery instead. */
+		if (unlikely(ret)) {
+			/*
+			 * -ENOMEM from the GFP_NOWAIT fallback is expected.
+			 * Anything else is a BUG(), but trigger recovery instead.
+			 */
+			WARN_ON_ONCE(ret != -ENOMEM);
 			goto recovery;
+		}
 
 		/* Allocated area. */
 		va = vas[area];
-- 
2.34.1