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?
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.
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.
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?
Honza
>
> The core of the issue is that while there is isolation between faults to
> the same range in the DAX fault handlers via our DAX entry locking, there
> is no isolation between faults in the code in mm/memory.c. This means for
> instance that this code in __handle_mm_fault() can run:
>
> if (pmd_none(*vmf.pmd) && transparent_hugepage_enabled(vma)) {
> ret = create_huge_pmd(&vmf);
>
> But by the time we actually get to run the fault handler called by
> create_huge_pmd(), the PMD is no longer pmd_none() because a racing PTE
> fault has installed a normal PMD here as a parent. This is the cause of
> the 2nd race. The first race is similar - there is the following check in
> handle_pte_fault():
>
> } else {
> /* See comment in pte_alloc_one_map() */
> if (pmd_devmap(*vmf->pmd) || pmd_trans_unstable(vmf->pmd))
> return 0;
>
> So if a pmd_devmap() PMD (a DAX PMD) has been installed at vmf->pmd, we
> will bail and retry the fault. This is correct, but there is nothing
> preventing the PMD from being installed after this check but before we
> actually get to the DAX PTE fault handlers.
>
> In my testing these races result in the following types of errors:
>
> BUG: Bad rss-counter state mm:ffff8800a817d280 idx:1 val:1
> BUG: non-zero nr_ptes on freeing mm: 15
>
> Fix this issue by having the DAX fault handlers verify that it is safe to
> continue their fault after they have taken an entry lock to block other
> racing faults.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Reported-by: Pawel Lebioda <pawel.lebioda@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
>
> ---
>
> I've written a new xfstest for this race, which I will send in response to
> this patch series. This series has also survived an xfstest run without
> any new issues.
>
> ---
> fs/dax.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index c22eaf1..3cc02d1 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1155,6 +1155,15 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf,
> }
>
> /*
> + * It is possible, particularly with mixed reads & writes to private
> + * mappings, that we have raced with a PMD fault that overlaps with
> + * the PTE we need to set up. Now that we have a locked mapping entry
> + * we can safely unmap the huge PMD so that we can install our PTE in
> + * our page tables.
> + */
> + split_huge_pmd(vmf->vma, vmf->pmd, vmf->address);
> +
> + /*
> * Note that we don't bother to use iomap_apply here: DAX required
> * the file system block size to be equal the page size, which means
> * that we never have to deal with more than a single extent here.
> @@ -1398,6 +1407,15 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
> goto fallback;
>
> /*
> + * It is possible, particularly with mixed reads & writes to private
> + * mappings, that we have raced with a PTE fault that overlaps with
> + * the PMD we need to set up. If so we just fall back to a PTE fault
> + * ourselves.
> + */
> + if (!pmd_none(*vmf->pmd))
> + goto unlock_entry;
> +
> + /*
> * Note that we don't use iomap_apply here. We aren't doing I/O, only
> * setting up a mapping, so really we're using iomap_begin() as a way
> * to look up our filesystem block.
> --
> 2.9.4
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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