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

[David Hildenbrand]

On 12.06.24 00:21, Alex Williamson wrote:
> On Tue, 11 Jun 2024 17:37:20 +0200
> Niklas Schnelle <schnelle@xxxxxxxxxxxxx> 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,

I see us now using vmf_insert_pfn(), which should be the right thing to 
do. So I suspect this problem should be disappearing.

--
Cheers,

David / dhildenb