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

[Paul Mackerras]

On Wed, May 18, 2016 at 10:07:19AM +0200, Hannes Reinecke wrote:
> On 05/18/2016 12:19 AM, Dan Williams wrote:
> > On Tue, May 17, 2016 at 3:57 AM, Johannes Thumshirn <jthumshirn@xxxxxxx> 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@xxxxxxxxxx>
> >>> Cc: Christoph Hellwig <hch@xxxxxx>
> >>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >>> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> >>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> >>> ---
> >>>  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?

I think you want the other Paul, Paul McKenney.

Paul.