linux-kernel - [PATCH v2] usb: core: warn if a GFP zone flag is passed to hcd_buffer

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Message-ID: <20250325134000.575794-1-ptesarik@suse.com>
Date: Tue, 25 Mar 2025 14:40:00 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	Petr Tesarik <ptesarik@...e.com>
Subject: [PATCH v2] usb: core: warn if a GFP zone flag is passed to hcd_buffer_alloc()

Remove a misleading comment and issue a warning if a zone modifier is
specified when allocating a hcd buffer.

There is no valid use case for a GFP zone modifier in hcd_buffer_alloc():
- PIO mode can use any kernel-addressable memory
- dma_alloc_coherent() ignores memory zone bits

This function is called by usb_alloc_coherent() and indirectly by
usb_submit_urb(). Despite the comment, no in-tree users currently pass
GFP_DMA.

Signed-off-by: Petr Tesarik <ptesarik@...e.com>
---
 drivers/usb/core/buffer.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/core/buffer.c b/drivers/usb/core/buffer.c
index 87230869e1fa..10844cd42e66 100644
--- a/drivers/usb/core/buffer.c
+++ b/drivers/usb/core/buffer.c
@@ -108,10 +108,6 @@ void hcd_buffer_destroy(struct usb_hcd *hcd)
 }
 
 
-/* sometimes alloc/free could use kmalloc with GFP_DMA, for
- * better sharing and to leverage mm/slab.c intelligence.
- */
-
 void *hcd_buffer_alloc(
 	struct usb_bus		*bus,
 	size_t			size,
@@ -128,6 +124,12 @@ void *hcd_buffer_alloc(
 	if (hcd->localmem_pool)
 		return gen_pool_dma_alloc(hcd->localmem_pool, size, dma);
 
+	/*
+	 * Zone modifiers are ignored by DMA API, and PIO should always use
+	 * GFP_KERNEL.
+	 */
+	WARN_ON_ONCE(mem_flags & GFP_ZONEMASK);
+
 	/* some USB hosts just use PIO */
 	if (!hcd_uses_dma(hcd)) {
 		*dma = ~(dma_addr_t) 0;
-- 
2.48.1