Re: [PATCH v2 2/3] /dev/dax, core: file operations and dax-mmap

[Hannes Reinecke <hare@suse.de>]

On 05/18/2016 12:19 AM, Dan Williams wrote:
> On Tue, May 17, 2016 at 3:57 AM, Johannes Thumshirn <jthumshirn@suse.de> wrote:
>> On Sat, May 14, 2016 at 11:26:29PM -0700, Dan Williams wrote:
>>> The "Device DAX" core enables dax mappings of performance / feature
>>> differentiated memory.  An open mapping or file handle keeps the backing
>>> struct device live, but new mappings are only possible while the device
>>> is enabled.   Faults are handled under rcu_read_lock to synchronize
>>> with the enabled state of the device.
>>>
>>> Similar to the filesystem-dax case the backing memory may optionally
>>> have struct page entries.  However, unlike fs-dax there is no support
>>> for private mappings, or mappings that are not backed by media (see
>>> use of zero-page in fs-dax).
>>>
>>> Mappings are always guaranteed to match the alignment of the dax_region.
>>> If the dax_region is configured to have a 2MB alignment, all mappings
>>> are guaranteed to be backed by a pmd entry.  Contrast this determinism
>>> with the fs-dax case where pmd mappings are opportunistic.  If userspace
>>> attempts to force a misaligned mapping, the driver will fail the mmap
>>> attempt.  See dax_dev_check_vma() for other scenarios that are rejected,
>>> like MAP_PRIVATE mappings.
>>>
>>> Cc: Jeff Moyer <jmoyer@redhat.com>
>>> Cc: Christoph Hellwig <hch@lst.de>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>>> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
>>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>>> ---
>>>  drivers/dax/Kconfig |    1
>>>  drivers/dax/dax.c   |  316 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>>  mm/huge_memory.c    |    1
>>>  mm/hugetlb.c        |    1
>>>  4 files changed, 319 insertions(+)
>>>
>>> diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
>>> index 86ffbaa891ad..cedab7572de3 100644
>>> --- a/drivers/dax/Kconfig
>>> +++ b/drivers/dax/Kconfig
>>> @@ -1,6 +1,7 @@
>>>  menuconfig DEV_DAX
>>>       tristate "DAX: direct access to differentiated memory"
>>>       default m if NVDIMM_DAX
>>> +     depends on TRANSPARENT_HUGEPAGE
>>>       help
>>>         Support raw access to differentiated (persistence, bandwidth,
>>>         latency...) memory via an mmap(2) capable character
>>> diff --git a/drivers/dax/dax.c b/drivers/dax/dax.c
>>> index 8207fb33a992..b2fe8a0ce866 100644
>>> --- a/drivers/dax/dax.c
>>> +++ b/drivers/dax/dax.c
>>> @@ -49,6 +49,7 @@ struct dax_region {
>>>   * @region - parent region
>>>   * @dev - device backing the character device
>>>   * @kref - enable this data to be tracked in filp->private_data
>>> + * @alive - !alive + rcu grace period == no new mappings can be established
>>>   * @id - child id in the region
>>>   * @num_resources - number of physical address extents in this device
>>>   * @res - array of physical address ranges
>>> @@ -57,6 +58,7 @@ struct dax_dev {
>>>       struct dax_region *region;
>>>       struct device *dev;
>>>       struct kref kref;
>>> +     bool alive;
>>>       int id;
>>>       int num_resources;
>>>       struct resource res[0];
>>> @@ -150,6 +152,10 @@ static void destroy_dax_dev(void *_dev)
>>>
>>>       dev_dbg(dev, "%s\n", __func__);
>>>
>>> +     /* disable and flush fault handlers, TODO unmap inodes */
>>> +     dax_dev->alive = false;
>>> +     synchronize_rcu();
>>> +
>>
>> IIRC RCU is only protecting a pointer, not the content of the pointer, so this
>> looks wrong to me.
> 
> The driver is using RCU to guarantee that all currently running fault
> handlers have either completed or will see the new state of ->alive
> when they start.  Reference counts are protecting the actual dax_dev
> object.
> 
Hmm.
This is the same 'creative' RCU usage Mike Snitzer has been trying
when trying to improve device-mapper performance.

>From my understanding RCU is protecting the _pointer_, not the
values of the structure pointed to.
IOW we are guaranteed to have a valid pointer at any time.
But at the same time _no_ guarantee is made about the _contents_ of
the structure.
It might well be that using 'synchronize_rcu' giving you similar
results (as synchronize_rcu() is essentially waiting a SMP grace
period, after which all CPUs should be seeing the update).
However, I haven't been able to find that this is a guaranteed
behaviour.
So from my understanding you have to use locking primitives
protecting the contents of the structure or exchange the _entire_
structure if you want to rely on RCU here.

Can we get some clarification here?
Paul?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
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