I've been trying to get mmap() working with the hyperv_fb.c fbdev driver, which is for Linux guests running on Microsoft's Hyper-V hypervisor. The hyperv_fb driver uses fbdev deferred I/O for performance reasons. But it looks to me like fbdev deferred I/O is fundamentally broken when the underlying framebuffer memory is allocated from kernel memory (alloc_pages or dma_alloc_coherent). The hyperv_fb.c driver may allocate the framebuffer memory in several ways, depending on the size of the framebuffer specified by the Hyper-V host and the VM "Generation". For a Generation 2 VM, the framebuffer memory is allocated by the Hyper-V host and is assigned to guest MMIO space. The hyperv_fb driver does a vmalloc() allocation for deferred I/O to work against. This combination handles mmap() of /dev/fb<n> correctly and the performance benefits of deferred I/O are substantial. But for a Generation 1 VM, the hyperv_fb driver allocates the framebuffer memory in contiguous guest physical memory using alloc_pages() or dma_alloc_coherent(), and informs the Hyper-V host of the location. In this case, mmap() with deferred I/O does not work. The mmap() succeeds, and user space updates to the mmap'ed memory are correctly reflected to the framebuffer. But when the user space program does munmap() or terminates, the Linux kernel free lists become scrambled and the kernel eventually panics. The problem is that when munmap() is done, the PTEs in the VMA are cleaned up, and the corresponding struct page refcounts are decremented. If the refcount goes to zero (which it typically will), the page is immediately freed. In this way, some or all of the framebuffer memory gets erroneously freed. From what I see, the VMA should be marked VM_PFNMAP when allocated memory kernel is being used as the framebuffer with deferred I/O, but that's not happening. The handling of deferred I/O page faults would also need updating to make this work. The fbdev deferred I/O support was originally added to the hyperv_fb driver in the 5.6 kernel, and based on my recent experiments, it has never worked correctly when the framebuffer is allocated from kernel memory. fbdev deferred I/O support for using kernel memory as the framebuffer was originally added in commit 37b4837959cb9 back in 2008 in Linux 2.6.29. But I don't see how it ever worked properly, unless changes in generic memory management somehow broke it in the intervening years. I think I know how to fix all this. But before working on a patch, I wanted to check with the fbdev community to see if this might be a known issue and whether there is any additional insight someone might offer. Thanks for any comments or help. Michael Kelley
