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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 11 Jun 2024 17:37:20 +0200
Niklas Schnelle <schnelle@linux.ibm.com> wrote:

> On Tue, 2024-06-11 at 17:10 +0200, David Hildenbrand wrote:
> > > > 
> > > > 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?  
> > 
> > Well, if you're calling remap_pfn_range() right now the expectation is 
> > that we hold it in write mode. :)
> > 
> > Staring at some random users, they all call it from mmap(), where you 
> > hold the mmap lock in write mode.
> > 
> > 
> > I wonder why we are not seeing that splat with vfio all of the time?
> > 
> > That mmap lock check was added "recently". In 1c71222e5f23 we started 
> > using vm_flags_set(). That (including the mmap_assert_write_locked()) 
> > check was added via bc292ab00f6c almost 1.5 years ago.
> > 
> > Maybe vfio is a bit special and was never really run with lockdep?
> >   
> > >   
> > > > 
> > > > 
> > > > 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?
> > >   
> > 
> > I think we should see it there as well, unless I am missing something.  
> 
> Well good news for me, bad news for everyone else. I just reproduced
> the same problem on my x86_64 workstation. I "ported over" (hacked it
> until it compiles) an x86 version of my trivial vfio-pci user-space
> test code that mmaps() the BAR 0 of an NVMe and MMIO reads the NVMe
> version field at offset 8. On my x86_64 box this leads to the following
> splat (still on v6.10-rc1).

There's already a fix for this queued[1] in my for-linus branch for
v6.10.  The problem has indeed existed with lockdep for some time but
only with the recent lockdep changes to generate a warning regardless
of debug kernel settings has it gone from just sketchy to having a fire
under it.  There's still an outstanding question of whether we
can/should insert as many pfns as we can during the fault[2] to reduce
the new overhead and hopefully at some point we'll have an even cleaner
option to use huge_fault for pfnmaps, but currently
vmf_insert_pfn_{pmd,pud} don't work with those pfnmaps.

So hopefully this problem disappears on current linux-next, but let me
know if there's still an issue.  Thanks,

Alex

[1]https://lore.kernel.org/all/20240530045236.1005864-1-alex.williamson@redhat.com/
[2]https://lore.kernel.org/all/20240607035213.2054226-1-alex.williamson@redhat.com/

> [  555.396773] ------------[ cut here ]------------
> [  555.396774] WARNING: CPU: 3 PID: 1424 at include/linux/rwsem.h:85 remap_pfn_range_notrack+0x625/0x650
> [  555.396778] Modules linked in: vfio_pci <-- 8< -->
> [  555.396877] CPU: 3 PID: 1424 Comm: vfio-test Tainted: G        W          6.10.0-rc1-niks-00007-gb19d6d864df1 #4 d09afec01ce27ca8218580af28295f25e2d2ed53
> [  555.396880] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./X570 Creator, BIOS P3.40 01/28/2021
> [  555.396881] RIP: 0010:remap_pfn_range_notrack+0x625/0x650
> [  555.396884] Code: a8 00 00 00 75 39 44 89 e0 48 81 c4 b0 00 00 00 5b 41 5c 41 5d 41 5e 41 5f 5d e9 26 a7 e5 00 cc 0f 0b 41 bc ea ff ff ff eb c9 <0f> 0b 49 8b 47 10 e9 72 fa ff ff e8 8b 56 b5 ff e9 c0 fa ff ff e8
> [  555.396887] RSP: 0000:ffffaf8b04ed3bc0 EFLAGS: 00010246
> [  555.396889] RAX: ffff9ea747cfe300 RBX: 00000000000ee200 RCX: 0000000000000100
> [  555.396890] RDX: 00000000000ee200 RSI: ffff9ea747cfe300 RDI: ffff9ea76db58fd0
> [  555.396892] RBP: 00000000ffffffea R08: 8000000000000035 R09: 0000000000000000
> [  555.396894] R10: ffff9ea76d9bbf40 R11: ffffffff96e5ce50 R12: 0000000000004000
> [  555.396895] R13: 00007f23b988a000 R14: ffff9ea76db58fd0 R15: ffff9ea76db58fd0
> [  555.396897] FS:  00007f23b9561740(0000) GS:ffff9eb66e780000(0000) knlGS:0000000000000000
> [  555.396899] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  555.396901] CR2: 00007f23b988a008 CR3: 0000000136bde000 CR4: 0000000000350ef0
> [  555.396903] Call Trace:
> [  555.396904]  <TASK>
> [  555.396905]  ? __warn+0x18c/0x2a0
> [  555.396908]  ? remap_pfn_range_notrack+0x625/0x650
> [  555.396911]  ? report_bug+0x1bb/0x270
> [  555.396915]  ? handle_bug+0x42/0x70
> [  555.396917]  ? exc_invalid_op+0x1a/0x50
> [  555.396920]  ? asm_exc_invalid_op+0x1a/0x20
> [  555.396923]  ? __pfx_is_ISA_range+0x10/0x10
> [  555.396926]  ? remap_pfn_range_notrack+0x625/0x650
> [  555.396929]  ? asm_exc_invalid_op+0x1a/0x20
> [  555.396933]  ? track_pfn_remap+0x170/0x180
> [  555.396936]  remap_pfn_range+0x6f/0xc0
> [  555.396940]  vfio_pci_mmap_fault+0xf3/0x1b0 [vfio_pci_core 6df3b7ac5dcecb63cb090734847a65c799a8fef2]
> [  555.396946]  __do_fault+0x11b/0x210
> [  555.396949]  do_pte_missing+0x239/0x1350
> [  555.396953]  handle_mm_fault+0xb10/0x18b0
> [  555.396959]  do_user_addr_fault+0x293/0x710
> [  555.396963]  exc_page_fault+0x82/0x1c0
> [  555.396966]  asm_exc_page_fault+0x26/0x30
> [  555.396968] RIP: 0033:0x55b0ea8bb7ac
> [  555.396972] Code: 00 00 b0 00 e8 e5 f8 ff ff 31 c0 48 83 c4 20 5d c3 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 48 89 7d f8 48 8b 45 f8 <8b> 00 89 c0 5d c3 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48
> [  555.396974] RSP: 002b:00007fff80973530 EFLAGS: 00010202
> [  555.396976] RAX: 00007f23b988a008 RBX: 00007fff80973738 RCX: 00007f23b988a000
> [  555.396978] RDX: 0000000000000001 RSI: 00007fff809735e8 RDI: 00007f23b988a008
> [  555.396979] RBP: 00007fff80973530 R08: 0000000000000005 R09: 0000000000000000
> [  555.396981] R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000002
> [  555.396982] R13: 0000000000000000 R14: 00007f23b98c8000 R15: 000055b0ea8bddc0
> [  555.396986]  </TASK>
> [  555.396987] ---[ end trace 0000000000000000 ]---
>