Messages in this thread | | | Subject | Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch | From | Jeremy Linton <> | Date | Mon, 4 May 2020 08:21:31 -0500 |
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Hi,
On 5/4/20 2:13 AM, Greg KH wrote: > On Fri, May 01, 2020 at 10:47:22AM -0500, Jeremy Linton wrote: >> Hi, >> >> Thanks for taking a look at this. >> >> On 5/1/20 2:05 AM, Greg KH wrote: >>> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote: >>>> On arm64, and possibly other architectures, requesting >>>> IO coherent memory may return Normal-NC if the underlying >>>> hardware isn't coherent. If these pages are then >>>> remapped into userspace as Normal, that defeats the >>>> purpose of getting Normal-NC, as well as resulting in >>>> mappings with differing cache attributes. >>> >>> What is "Normal-NC"? >> >> A non-cacheable attribute on arm64 pages. I think Mark R & Marc Z elaborated >> while I was asleep (thanks!). >> . >> >> >>> >>>> In particular this happens with libusb, when it attempts >>>> to create zero-copy buffers as is used by rtl-sdr, and >>> >>> What is "rtl-sdr" >> >> Its the realtek software defined radio (SDR), a really inexpensive TV dongle >> that was discovered could be used as a general purpose SDR a decade or so >> ago. In particular, this project >> https://github.com/osmocom/rtl-sdr/ >> which is packaged by fedora/etc. >> >>> >>>> maybe other applications. The result is usually >>>> application death. >>> >>> So is this a new problem? Old problem? Old problem only showing up on >>> future devices? On current devices? I need a hint here as to know if >>> this is a bugfix or just work to make future devices work properly. >> >> This has been a problem on arm devices without IO coherent USB apparently >> for years. The rtl-sdr project itself has a disable zero-copy mode that >> people have been using on rpi/etc specific builds. Fedora OTOH, is building >> it with the same flags on x86 & arm64 which means that people report >> problems. This happened a few days ago (on a pinebook), and I duplicated it >> on an NXP platform just running the `rtl_test` artifact with a nooelec from >> my junk box. Guessing that it was a page mismatch I went looking for that, >> rather than disabling the zero copy since punishing arm machine that have IO >> coherent USB adapters for the sins of these low end devices isn't ideal. I >> found this, and this patch allows the rtl_test app to run without issues on >> my NXP/solidrun. >> >> Plus, given that its actually a kernel/libusb problem its likely there are >> other applications having similar problems. >> >>> >>>> >>>> If dma_mmap_attr() is used instead of remap_pfn_range, >>>> the page cache/etc attributes can be matched between the >>>> kernel and userspace. >>>> >>>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com> >>>> --- >>>> drivers/usb/core/devio.c | 5 ++--- >>>> 1 file changed, 2 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c >>>> index 6833c918abce..1e7458dd6e5d 100644 >>>> --- a/drivers/usb/core/devio.c >>>> +++ b/drivers/usb/core/devio.c >>>> @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma) >>>> { >>>> struct usb_memory *usbm = NULL; >>>> struct usb_dev_state *ps = file->private_data; >>>> + struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus); >>>> size_t size = vma->vm_end - vma->vm_start; >>>> void *mem; >>>> unsigned long flags; >>>> @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma) >>>> usbm->vma_use_count = 1; >>>> INIT_LIST_HEAD(&usbm->memlist); >>>> - if (remap_pfn_range(vma, vma->vm_start, >>>> - virt_to_phys(usbm->mem) >> PAGE_SHIFT, >>>> - size, vma->vm_page_prot) < 0) { >>>> + if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) { >>> >>> Given that this code has not changed since 2016, how has no one noticed >>> this issue before? >> They have there are a lot of reports of sdr failures, but the general use >> case is rare? >> >>> >>> And have you tested this change out on other systems (i.e. x86) to >>> ensure that this still works properly? >> >> Yes and no, I did some basic libusb tests on an x86 machine, but its a bit >> tricky at the moment for me to get the rtl plugged into a x86 test machine. >> (its a work in progress). >> >> >>> >>> And why isn't this call used more by drivers if this is a real issue? >> The particulars of asking for iocoherent memory and then mapping it to >> userspace is rarer than just asking for kmalloc()/remap() and then >> performing the dma ops? >> >> Then there are all the softer issues around arm64 testing/availability and >> vendors carrying "fixes" for particular issues (like rtl-sdr disabling zero >> copy). >> >>> And will this cause issues with how the userspace mapping is handled as >>> now we rely on userspace to do things differently? Or am I reading the >>> dma_mmap_attrs() documentation wrong? >> I don't think userspace is doing anything differently here, and AFAIK, on >> systems with IO coherent adapters this ends up with the same page mapping as >> just doing the remap_pfn_rage() with the same attributes as before. I've >> looked at dma_map_attrs() a bit, but i'm also trusting it does what it says >> on the tin. >> >> >> Thanks again for looking at this. > > Ok, can I get a lot better written changelog text for this patch based > on this thread, so that it makes more sense when we merge this patch?
Yes, sure. I also plan on changing it to dma_mmap_coherent() which is the same as the dma_mmap_attrs() with the attrs as above.
I will re-post later this afternoon.
Thanks,
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