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

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Tue, 2024-06-11 at 13:21 +0200, Niklas Schnelle wrote:
> 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@nvidia.com>
> > Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
> > Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> > ---
> >  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 ]---
> 

This has me a bit confused so far as __is_vma_write_locked() checks
mmap_assert_write_locked(vma->vm_mm) but most other users of
fixup_user_fault() hold mmap_read_lock() just like this code and
clearly in the non page fault case we only need the read lock.