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

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

> > > 
--- 8< snip 8< ---
> > > > 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.
> 
> This is likely the 
> vm_flags_set()->vma_start_write(vma)->__is_vma_write_locked()

Yes

> 
> which checks mmap_assert_write_locked().
> 
> Setting VMA flags would be racy with the mmap lock in read mode.
> 
> 
> remap_pfn_range() documents: "this is only safe if the mm semaphore is 
> held when called." which doesn't spell out if it needs to be held in 
> write mode (which I think it does) :)

Logically this makes sense to me. At the same time it looks like
fixup_user_fault() expects the caller to only hold mmap_read_lock() as
I do here. In there it even retakes mmap_read_lock(). But then wouldn't
any fault handling by its nature need to hold the write lock?

> 
> 
> My best guess is: if you are using remap_pfn_range() from a fault 
> handler (not during mmap time) you are doing something wrong, that's why 
> you get that report.

@Alex: I guess so far the vfio_pci_mmap_fault() handler is only ever
triggered by "normal"/"actual" page faults where this isn't a problem?
Or could it be a problem there too?

> 
> vmf_insert_pfn() and friends might be better alternatives, that make 
> sure that the VMA already received the proper VMA flags at mmap time.
> 
> > > 
> > 
> > And it gets weirder, as I could have sworn that I properly tested this
> > on v1, I retested with v1 (tags/sent/vfio_pci_mmap-v1 on my
> > git.kernel.org/niks and based on v6.9) and there I don't get the above
> > warning. I also made sure that it's not caused by my change to
> > "current->mm" for v2. But I'm also not hitting the checks David moved
> > into follow_pte() so yeah not sure what's going on here.
> 
> 
> You mean the mmap_assert_locked()? Yeah, that only checks if you have it 
> in read mode, but not in write mode.
> 

Turns out this part was just me being stupid and not running the old
version with lockdep enabled when it "worked" and this only turned into
a normal warn with commit ba168b52bf8e ("mm: use rwsem assertion macros
for mmap_lock"). Rerunning v1 with lockdep on gave me an equivalent
lockdep splat.