[PATCH v4 -next] cma: Enforce non-zero pageblock_order during cma_init_reserved_mem()

cma_init_reserved_mem() checks base and size alignment with
CMA_MIN_ALIGNMENT_BYTES. However, some users might call this during
early boot when pageblock_order is 0. That means if base and size does
not have pageblock_order alignment, it can cause functional failures
during cma activate area.

So let's enforce pageblock_order to be non-zero during
cma_init_reserved_mem() to catch such wrong usages.

Acked-by: David Hildenbrand <redacted>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Anshuman Khandual <redacted>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
RFCv3 -> v4:
1. Dropped RFC tagged as requested by Andrew.
2. Updated the changelog & added background as requested by Andrew [RFCv3]
3. Added acked-by and RBs tags.
4. Small commit msg update.
5. No functional changes.
[RFCv3]: https://lore.kernel.org/all/20241112225902.f20215e5015f4d7cdf502302@linux-foundation.org/ (local)

Background -
============
1. This was seen with fadump on PowerPC which was calling
   cma_init_reserved_mem() before the pageblock_order was initialized.
   This is now fixed in the fadump on PowerPC itself. The details of that
   can be found in the patch including the userspace-visible effect of the issue [1].
2. However it was also decided that we should add a stronger enforcement check
   within cma_init_reserved_mem() to catch such wrong usages [2]. Hence this
   patch. This is ok to be in -next and there is no "Fixes" tag required for
   this patch.

[1]: https://lore.kernel.org/all/3ae208e48c0d9cefe53d2dc4f593388067405b7d.1729146153.git.ritesh.list@gmail.com/ (local)
[2]: https://lore.kernel.org/all/83eb128e-4f06-4725-a843-a4563f246a44@redhat.com/ (local)

 mm/cma.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/cma.c b/mm/cma.c
index 3e9724716bad..36d753e7a0bf 100644
--- a/mm/cma.c
+++ b/mm/cma.c

@@ -182,6 +182,15 @@ int __init cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
 	if (!size || !memblock_is_region_reserved(base, size))
 		return -EINVAL;

+	/*
+	 * CMA uses CMA_MIN_ALIGNMENT_BYTES as alignment requirement which
+	 * needs pageblock_order to be initialized. Let's enforce it.
+	 */
+	if (!pageblock_order) {
+		pr_err("pageblock_order not yet initialized. Called during early boot?\n");
+		return -EINVAL;
+	}
+
 	/* ensure minimal alignment required by mm core */
 	if (!IS_ALIGNED(base | size, CMA_MIN_ALIGNMENT_BYTES))
 		return -EINVAL;
--

2.46.0