Re: [PATCH v3 1/3] s390/pci: Fix s390_mmio_read/write syscall page fault handling

[Niklas Schnelle]

On Wed, 2024-05-29 at 13:36 +0200, Niklas Schnelle wrote:
> The s390 MMIO syscalls when using the classic PCI instructions do not
> cause a page fault when follow_pte() fails due to the page not being
> present. Besides being a general deficiency this breaks vfio-pci's mmap()
> handling once VFIO_PCI_MMAP gets enabled as this lazily maps on first
> access. Fix this by following a failed follow_pte() with
> fixup_user_page() and retrying the follow_pte().
> 
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Reviewed-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> ---
>  arch/s390/pci/pci_mmio.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
> index 5398729bfe1b..80c21b1a101c 100644
> --- a/arch/s390/pci/pci_mmio.c
> +++ b/arch/s390/pci/pci_mmio.c
> @@ -170,8 +170,12 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
>  		goto out_unlock_mmap;
>  
>  	ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
> -	if (ret)
> -		goto out_unlock_mmap;
> +	if (ret) {
> +		fixup_user_fault(current->mm, mmio_addr, FAULT_FLAG_WRITE, NULL);
> +		ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
> +		if (ret)
> +			goto out_unlock_mmap;
> +	}
>  
>  	io_addr = (void __iomem *)((pte_pfn(*ptep) << PAGE_SHIFT) |
>  			(mmio_addr & ~PAGE_MASK));
> @@ -305,12 +309,16 @@ SYSCALL_DEFINE3(s390_pci_mmio_read, unsigned long, mmio_addr,
>  	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
>  		goto out_unlock_mmap;
>  	ret = -EACCES;
> -	if (!(vma->vm_flags & VM_WRITE))
> +	if (!(vma->vm_flags & VM_READ))
>  		goto out_unlock_mmap;
>  
>  	ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
> -	if (ret)
> -		goto out_unlock_mmap;
> +	if (ret) {
> +		fixup_user_fault(current->mm, mmio_addr, 0, NULL);
> +		ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
> +		if (ret)
> +			goto out_unlock_mmap;
> +	}
>  
>  	io_addr = (void __iomem *)((pte_pfn(*ptep) << PAGE_SHIFT) |
>  			(mmio_addr & ~PAGE_MASK));
> 

Ughh, I think I just stumbled over a problem with this. This is a
failing lock held assertion via __is_vma_write_locked() in
remap_pfn_range_notrack() but I'm not sure yet what exactly causes this

[   67.338855] ------------[ cut here ]------------
[   67.338865] WARNING: CPU: 15 PID: 2056 at include/linux/rwsem.h:85 remap_pfn_range_notrack+0x596/0x5b0
[   67.338874] Modules linked in: <--- 8< --->
[   67.338931] CPU: 15 PID: 2056 Comm: vfio-test Not tainted 6.10.0-rc1-pci-pfault-00004-g193e3a513cee #5
[   67.338934] Hardware name: IBM 3931 A01 701 (LPAR)
[   67.338935] Krnl PSW : 0704c00180000000 000003e54c9730ea (remap_pfn_range_notrack+0x59a/0x5b0)
[   67.338940]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
[   67.338944] Krnl GPRS: 0000000000000100 000003655915fb78 000002d80b9a5928 000003ff7fa00000
[   67.338946]            0004008000000000 0000000000004000 0000000000000711 000003ff7fa04000
[   67.338948]            000002d80c533f00 000002d800000100 000002d81bbe6c28 000002d80b9a5928
[   67.338950]            000003ff7fa00000 000002d80c533f00 000003e54c973120 000003655915fab0
[   67.338956] Krnl Code: 000003e54c9730de: a708ffea            lhi     %r0,-22
                          000003e54c9730e2: a7f4fff6            brc     15,000003e54c9730ce
                         #000003e54c9730e6: af000000            mc      0,0
                         >000003e54c9730ea: a7f4fd6e            brc     15,000003e54c972bc6
                          000003e54c9730ee: af000000            mc      0,0
                          000003e54c9730f2: af000000            mc      0,0
                          000003e54c9730f6: 0707                bcr     0,%r7
                          000003e54c9730f8: 0707                bcr     0,%r7
[   67.339025] Call Trace:
[   67.339027]  [<000003e54c9730ea>] remap_pfn_range_notrack+0x59a/0x5b0
[   67.339032]  [<000003e54c973120>] remap_pfn_range+0x20/0x30
[   67.339035]  [<000003e4cce5396c>] vfio_pci_mmap_fault+0xec/0x1d0 [vfio_pci_core]
[   67.339043]  [<000003e54c977240>] handle_mm_fault+0x6b0/0x25a0
[   67.339046]  [<000003e54c966328>] fixup_user_fault+0x138/0x310
[   67.339048]  [<000003e54c63a91c>] __s390x_sys_s390_pci_mmio_read+0x28c/0x3a0
[   67.339051]  [<000003e54c5e200a>] do_syscall+0xea/0x120
[   67.339055]  [<000003e54d5f9954>] __do_syscall+0x94/0x140
[   67.339059]  [<000003e54d611020>] system_call+0x70/0xa0
[   67.339063] Last Breaking-Event-Address:
[   67.339065]  [<000003e54c972bc2>] remap_pfn_range_notrack+0x72/0x5b0
[   67.339067] ---[ end trace 0000000000000000 ]---