Re: [PATCH 2/4] usb: usbfs: Use consistent mmap functions

From: David Hildenbrand
Date: Wed May 10 2023 - 12:35:11 EST

On 10.05.23 17:41, Ruihan Li wrote:
Hi Alan,

On Wed, May 10, 2023 at 10:38:48AM -0400, Alan Stern wrote:
On Wed, May 10, 2023 at 04:55:25PM +0800, Ruihan Li wrote:
When hcd->localmem_pool is non-null, it is used to allocate DMA memory.
In this case, the dma address will be properly returned (in dma_handle),
and dma_mmap_coherent should be used to map this memory into the user
space. However, the current implementation uses pfn_remap_range, which
is supposed to map normal pages (instead of DMA pages).

Instead of repeating the logic in the memory allocation function, this
patch introduces a more robust solution. To address the previous issue,
this patch checks the type of allocated memory by testing whether
dma_handle is properly set. If dma_handle is properly returned, it means
some DMA pages are allocated and dma_mmap_coherent should be used to map
them. Otherwise, normal pages are allocated and pfn_remap_range should
be called. This ensures that the correct mmap functions are used
consistently, independently with logic details that determine which type
of memory gets allocated.

Fixes: a0e710a7def4 ("USB: usbfs: fix mmap dma mismatch")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ruihan Li <lrh2000@xxxxxxxxxx>
drivers/usb/core/devio.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index b4cf9e860..5067030b7 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -235,7 +235,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
size_t size = vma->vm_end - vma->vm_start;
void *mem;
unsigned long flags;
- dma_addr_t dma_handle;
+ dma_addr_t dma_handle = DMA_MAPPING_ERROR;
int ret;
ret = usbfs_increase_memory_usage(size + sizeof(struct usb_memory));
@@ -265,7 +265,13 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
usbm->vma_use_count = 1;
- if (hcd->localmem_pool || !hcd_uses_dma(hcd)) {
+ /* In DMA-unavailable cases, hcd_buffer_alloc_pages allocates
+ * normal pages and assigns DMA_MAPPING_ERROR to dma_handle. Check
+ * whether we are in such cases, and then use remap_pfn_range (or
+ * dma_mmap_coherent) to map normal (or DMA) pages into the user
+ * space, respectively.
+ */

Another stylistic issue. For multi-line comments, the format we use is:

* Blah, blah, blah
* Blah, blah, blah

Alan Stern

Yeah, I am pretty sure it is another style difference with the net
subsystem. Again, in the next version, I'll follow the coding style that
you have pointed out.


spells out that net/ and drivers/net/ are "special".

Regarding breaking long lines, it's just an inconsistent, undocumented mess IIRC ...


David / dhildenb