Messages in this thread | | | Date | Wed, 26 Apr 2006 20:27:23 +1000 | From | Nick Piggin <> | Subject | Re: Using remap_pfn_range causes system hang on app close in 2.6.15 & up |
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Sam Abu-Nassar wrote: > Hello, > > I posted this query a couple of weeks ago regarding a problem with > remap_pfn_range. I was able to resolve the issue and I thought I would > post my findings in case it helps someone else or results in a kernel > fix. I will try to keep this short. > > In a nutshell, my driver support user APIs that maps system RAM or a PCI > BAR space to user space. What I did not mention in my original post is > that my driver performed a sort of custom protocol when mmap() is > called. Since mmap() really only provides a limited amount of space > (the offset field) that I can use to pass additional information to my > driver, I had implemented a custom protocol, which works as follows: > > 1. API calls mmap to obtain a user virtual address > 2. Drivers mmap routine stores the VMA in an internal list and returns ok. > 3. API then issues a custom IOCTL to driver to complete mapping with > additional info > 4. Driver retrieves VMA from internal list and performs mapping with > io/remap_pfn_range, depending upon whether it's to system RAM or a PCI BAR. > > The mappings always work fine, but starting with 2.6.15, the system > freeezes when the file descriptor is closed. I tried numerous tests and > compared my code with existing drivers, such as /dev/mem. Here is what > I found: > > The fix involved moving my calls to io/remap_pfn_range into my > Dispatch_mmap() routine. Once I did this, the system no longer > crashed. I still implement sending some custom information to the > driver, but now I use special values in the offset field, remembering > that the offset is eventually shifted by PAGE_SIZE by the time it > reaches the driver. My driver code essentially did not change. In > effect, all I really did was move it to the driver's mmap() routine. > > I should mention that my original protocol has worked fine in kernels > 2.2, 2.4, and up to 2.6.14. Some change to the VM subsystem in 2.6.15 > broke my original code. I don't believe there should be an issue with > calling remap_pfn_range outside of the driver's mmap() routine, but I am > not a kernel developer, so I could be wrong in my assumption. One of my > customers posed this question to Nick Piggin, and he seemed to think > there should not be a problem with this.
Well, I think I said it shouldn't oops like this... I don't think it is particularly robust WRT error cases or concurrent page faults (between mmap and ioctl).
As we established earlier with a debug patch, the reason for the oops is that VM_PFNMAP has been cleared from your vma->vm_flags for some reason. This is causing the unmap code to mistakenly try to remove reverse maps and refcounts from the struct pages.
I don't know why VM_PFNMAP should be getting cleared. But if it remains set then the oops should go away.
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