On Thu, Mar 9, 2017 at 4:11 PM, Dave Jiang <[email protected]> wrote:
>
>
> On 03/09/2017 05:03 PM, Dan Williams wrote:
>> On Thu, Mar 9, 2017 at 3:56 PM, Dave Jiang <[email protected]> wrote:
>>> Jeff Moyer reports that:
>>> "
>>> With a device dax alignment of 4KB or 2MB, I get sigbus when running the
>>> attached fio job file for the current kernel (4.11.0-rc1+). If I
>>> specify an alignment of 1GB, it works.
>>>
>>> I turned on debug output, and saw that it was failing in the huge fault
>>> code.
>>>
>>> [ 4614.138357] dax dax1.0: dax_open
>>> [ 4614.154838] dax dax1.0: dax_mmap
>>> [ 4614.171898] dax dax1.0: dax_dev_huge_fault: fio: write (0x7f08f0a00000 -
>>> 0x7f0ce0800000)
>>> [ 4614.211720] dax dax1.0: __dax_dev_pud_fault:
>>> phys_to_pgoff(0xffffffffcf600) failed
>>> [ 4614.568911] dax dax1.0: dax_release
>>>
>>> fio config for reproduce:
>>> [global]
>>> ioengine=dev-dax
>>> direct=0
>>> filename=/dev/dax0.0
>>> bs=2m
>>>
>>> [write]
>>> rw=write
>>>
>>> [read]
>>> stonewall
>>> rw=read
>>> "
>>>
>>> It looks like the code does not fallback at all when handling faults. The
>>> fault handlers are missing some boundary checking for the VMA. Also, we need
>>> to fall back if the alignment requested is less than the P-entry size.
>>>
>>> Reported-by: Jeff Moyer <[email protected]>
>>> Signed-off-by: Dave Jiang <[email protected]>
>>> ---
>>> drivers/dax/dax.c | 22 ++++++++++++++++++++--
>>> 1 file changed, 20 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/dax/dax.c b/drivers/dax/dax.c
>>> index 174690a..5c7998e 100644
>>> --- a/drivers/dax/dax.c
>>> +++ b/drivers/dax/dax.c
>>> @@ -480,12 +480,21 @@ static int __dax_dev_pmd_fault(struct dax_dev
>>> *dax_dev, struct vm_fault *vmf)
>>> return VM_FAULT_SIGBUS;
>>> }
>>>
>>> + /* fall back if we are outside of the VMA */
>>> + if (pmd_addr < vmf->vma->vm_start)
>>> + return VM_FAULT_FALLBACK;
>>> + if ((pmd_addr + PMD_SIZE) > vmf->vma->vm_end)
>>> + return VM_FAULT_FALLBACK;
>>
>> I think these are already handled by check_vma(), and we only want to
>> fallback if dax_region->align < PMD_SIZE, otherwise we're violating
>> device-dax's guaranteed fault granularity.
>
> I was seeing pud_addr < vmf->vma->vm_start in the pud handler. It
> doesn't look like check_vma() checks the boundaries.
Sorry, yes, we do need to do the vma boundary checks in
__dax_dev_{pmd,pud}_fault() since check_vma() does not know the fault
size. We just can't fallback blindly.
> Also, it wouldn't
> know the pud_addr or pmd_addr. Or are you saying it doesn't matter?
If the fault does not align to the vma we need to fall back, but only
if the current fault size is > dax_region->align and the fallback
fault size is >= dax_region->align.
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