Replace WARN_ON with WARN_ONCE when detecting attempts to ioremap
RAM. This prevents log spam when a misbehaving driver repeatedly tries
to map RAM via ioremap.

A single warning is more than enough to show the broken code path, and
extra reports don't add extra information.

Warning floods have been seen in production environments where broken
external drivers hit this code path thousand of times, causing
unnecessary messages to be printed and pressure on the serial console.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
PS: This is crashing a host due to a lockup on nbcon that is likely
fixed by commit d5d399efff65773("printk/nbcon: Release nbcon consoles ownership
in atomic flush after each emitted record")
---
 arch/arm64/mm/ioremap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/ioremap.c b/arch/arm64/mm/ioremap.c
index 10e246f11271..b12cbed9b5ad 100644
--- a/arch/arm64/mm/ioremap.c
+++ b/arch/arm64/mm/ioremap.c
@@ -24,7 +24,8 @@ void __iomem *ioremap_prot(phys_addr_t phys_addr, size_t size,
 		return NULL;
 
 	/* Don't allow RAM to be mapped. */
-	if (WARN_ON(pfn_is_map_memory(__phys_to_pfn(phys_addr))))
+	if (WARN_ONCE(pfn_is_map_memory(__phys_to_pfn(phys_addr)),
+		      "ioremap attempted on RAM pfn\n"))
 		return NULL;
 
 	/*

---
base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
change-id: 20251222-arm64_ioremap-d20703b85358

Best regards,
--  
Breno Leitao <leitao@debian.org>

On Mon, 22 Dec 2025 07:28:55 -0800, Breno Leitao wrote:
> Replace WARN_ON with WARN_ONCE when detecting attempts to ioremap
> RAM. This prevents log spam when a misbehaving driver repeatedly tries
> to map RAM via ioremap.
> 
> A single warning is more than enough to show the broken code path, and
> extra reports don't add extra information.
> 
> [...]

Applied to arm64 (for-next/misc), thanks!

[1/1] arm64: mm: warn once for ioremap attempts on RAM mappings
      https://git.kernel.org/arm64/c/53f45f045cbe

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

On Mon, Dec 22, 2025 at 07:28:55AM -0800, Breno Leitao wrote:
> Replace WARN_ON with WARN_ONCE when detecting attempts to ioremap
> RAM. This prevents log spam when a misbehaving driver repeatedly tries
> to map RAM via ioremap.
> 
> A single warning is more than enough to show the broken code path, and
> extra reports don't add extra information.
> 
> Warning floods have been seen in production environments where broken
> external drivers hit this code path thousand of times, causing
> unnecessary messages to be printed and pressure on the serial console.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>

It looks fine to me but it's not urgent for 6.19, so I'll leave it to
Will.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>