Re: [PATCH 2/2] dax: Fix race between colliding PMD & PTE entries

[Jan Kara]

On Thu 18-05-17 15:29:39, Ross Zwisler wrote:
> On Thu, May 18, 2017 at 09:50:37AM +0200, Jan Kara wrote:
> > On Wed 17-05-17 11:16:39, Ross Zwisler wrote:
> > > We currently have two related PMD vs PTE races in the DAX code.  These can
> > > both be easily triggered by having two threads reading and writing
> > > simultaneously to the same private mapping, with the key being that private
> > > mapping reads can be handled with PMDs but private mapping writes are
> > > always handled with PTEs so that we can COW.
> > > 
> > > Here is the first race:
> > > 
> > > CPU 0					CPU 1
> > > 
> > > (private mapping write)
> > > __handle_mm_fault()
> > >   create_huge_pmd() - FALLBACK
> > >   handle_pte_fault()
> > >     passes check for pmd_devmap()
> > > 
> > > 					(private mapping read)
> > > 					__handle_mm_fault()
> > > 					  create_huge_pmd()
> > > 					    dax_iomap_pmd_fault() inserts PMD
> > > 
> > >     dax_iomap_pte_fault() does a PTE fault, but we already have a DAX PMD
> > >     			  installed in our page tables at this spot.
> > >
> > > 
> > > Here's the second race:
> > > 
> > > CPU 0					CPU 1
> > > 
> > > (private mapping write)
> > > __handle_mm_fault()
> > >   create_huge_pmd() - FALLBACK
> > > 					(private mapping read)
> > > 					__handle_mm_fault()
> > > 					  passes check for pmd_none()
> > > 					  create_huge_pmd()
> > > 
> > >   handle_pte_fault()
> > >     dax_iomap_pte_fault() inserts PTE
> > > 					    dax_iomap_pmd_fault() inserts PMD,
> > > 					       but we already have a PTE at
> > > 					       this spot.
> > 
> > So I don't see how this second scenario can happen. dax_iomap_pmd_fault()
> > will call grab_mapping_entry(). That will either find PTE entry in the
> > radix tree -> EEXIST and we retry the fault. Or we will not find PTE entry
> > -> try to insert PMD entry which collides with the PTE entry -> EEXIST and
> > we retry the fault. Am I missing something?
> 
> Yep, sorry, I guess I needed a few extra steps in my flow (the initial private
> mapping read by CPU 0):
> 
> 
> CPU 0					CPU 1
> 
> (private mapping read)
> __handle_mm_fault()
>   passes check for pmd_none()
>   create_huge_pmd()
>     dax_iomap_pmd_fault() inserts PMD
> 
> (private mapping write)
> __handle_mm_fault()
>   create_huge_pmd() - FALLBACK
> 					(private mapping read)
> 					__handle_mm_fault()
> 					  passes check for pmd_none()
> 					  create_huge_pmd()
> 
>   handle_pte_fault()
>     dax_iomap_pte_fault() inserts PTE
> 					    dax_iomap_pmd_fault() inserts PMD,
> 					       but we already have a PTE at
> 					       this spot.
> 
> So what happens is that CPU 0 inserts a DAX PMD into the radix tree that has
> real storage backing, and all PTE and PMD faults just use that same PMD radix
> tree entry for locking and dirty tracking.

OK, I see now. So essentially it's the same catch as the other case -
grab_mapping_entry() returns PMD entry on CPU0 although we asked for PTE
entry.

> > The first scenario seems to be possible. dax_iomap_pmd_fault() will create
> > PMD entry in the radix tree. Then dax_iomap_pte_fault() will come, do
> > grab_mapping_entry(), there it sees entry is PMD but we are doing PTE fault
> > so I'd think that pmd_downgrade = true... But actually the condition there
> > doesn't trigger in this case. And that's a catch that although we asked
> > grab_mapping_entry() for PTE, we've got PMD back and that screws us later.
> 
> Yep, it was a concious decision when implementing the PMD support to allow one
> thread to use PMDs and another to use PTEs in the same range, as long as the
> thread faulting in PMDs is the first to insert into the radix tree.  A PMD
> radix tree entry will be inserted and used for locking and dirty tracking, and
> each thread or process can fault in either PTEs or PMDs into its own address
> space as needed.

Well, for *threads* it doesn't really make good sense to mix PMDs and PTEs
as they share page tables. However for *processes* it makes some sense to
allow one process to use PTEs and another process to use PMDs. And I
remember we were discussing this in the past.

> We can revisit this, if you think it is incorrect.  The option you outline
> below would basically mean that if any thread were to fault in a PTE in a
> range, all threads and processes would be forced to use PTEs because we would
> use PTEs in the radix tree.

Well, I don't think it is necessarily incorrect. I just think it is more
difficult to get it right (as current bugs show) so I'm just considering
whether the complexity is worth it.

> This is cleaner...I'm not sure if the use case of having two threads accessing
> the same area, one with PTEs and one with PMDs, is actually prevalent.  It's
> also maybe a bit weird that the current behavior varies based on which thread
> faulted first - if the PTE thread faults first, it'll insert a PTE into the
> radix tree and everyone will just use PTEs.

So for two *threads*, I don't think that is a sensible use-case. We just
have to get it right. For two *processes* it makes sense - your DB might
want to use PMDs while your backup program may just use PTEs. So thinking
more about it I guess it is worth the effort to make the mixed case work
efficiently.

> > Actually I'm not convinced your patch quite fixes this because
> > dax_load_hole() or dax_insert_mapping_entry() will modify the passed entry
> > with the assumption that it's PTE entry and so they will likely corrupt the
> > entry in the radix tree.
> 
> I don't think we can ever call dax_load_hole() if we have a DAX PMD entry in
> the radix tree, because we have a block mapping from the filesystem.
> 
> For dax_insert_mapping_entry(), we do the right thing.  From the comments
> above the function:
> 
>  * If we happen to be trying to insert a PTE and there is a PMD
>  * already in the tree, we will skip the insertion and just dirty the PMD as
>  * appropriate.  If we happen to be trying to insert a PTE and there is a PMD
>  * already in the tree, we will skip the insertion and just dirty the PMD as
>  * appropriate.

Yeah, on the first reading I missed that we won't modify the radix tree in
that particular case. Frankly, I think we should somewhat clean up that
code to make things more obvious but let's leave that for a bit later. For
now the code looks correct.

> > So I think to fix the first case we should rather modify
> > grab_mapping_entry() to properly go through the pmd_downgrade path once we
> > find PMD entry and we do PTE fault.
> > 
> > What do you think?
> 
> That could also work, though I do think the fix as submitted is correct.
> I think it comes down to whether we want to keep the behavior where a thread
> faulting in a PTEs will use an existing PMD entry in the radix tree, instead
> of making all other threads fall back to PTEs.
> 
> I think either way solves this issue for the DAX case...but do you understand
> how this is solved for other fault handlers?  They don't have any isolation
> between faults either in the mm/memory.c code, and are susceptible to the same
> races.  How do they deal with the fact that by the time they get to their PTE
> fault handler, a racing PMD fault handler in another thread could have
> inserted a PMD into their page tables, and vice versa?

So normal fault path uses alloc_set_pte() for installing new PTE. And that
uses pte_alloc_one_map() which checks whether PMD is still suitable for
inserting a PTE. If not, we return VM_FAULT_NOPAGE. Probably it would be
cleanest to factor our common parts of PTE and PMD insertion so that we can
use these functions both from DAX and generic fault paths.

Anyway, I'll have a look at your fixes with fresh eyes as they could be the
right way to go as a quick fix. Refactoring and cleanups can come later.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR