Re: [PATCH 2/3] mm, dax, pmem: Introduce dev_pagemap_failure()

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Mar 17, 2021 at 9:45 PM Darrick J. Wong <djwong@kernel.org> wrote:
>
> On Wed, Mar 17, 2021 at 09:08:23PM -0700, Dan Williams wrote:
> > Jason wondered why the get_user_pages_fast() path takes references on a
> > @pgmap object. The rationale was to protect against accessing a 'struct
> > page' that might be in the process of being removed by the driver, but
> > he rightly points out that should be solved the same way all gup-fast
> > synchronization is solved which is invalidate the mapping and let the
> > gup slow path do @pgmap synchronization [1].
> >
> > To achieve that it means that new user mappings need to stop being
> > created and all existing user mappings need to be invalidated.
> >
> > For device-dax this is already the case as kill_dax() prevents future
> > faults from installing a pte, and the single device-dax inode
> > address_space can be trivially unmapped.
> >
> > The situation is different for filesystem-dax where device pages could
> > be mapped by any number of inode address_space instances. An initial
> > thought was to treat the device removal event like a drop_pagecache_sb()
> > event that walks superblocks and unmaps all inodes. However, Dave points
> > out that it is not just the filesystem user-mappings that need to react
> > to global DAX page-unmap events, it is also filesystem metadata
> > (proposed DAX metadata access), and other drivers (upstream
> > DM-writecache) that need to react to this event [2].
> >
> > The only kernel facility that is meant to globally broadcast the loss of
> > a page (via corruption or surprise remove) is memory_failure(). The
> > downside of memory_failure() is that it is a pfn-at-a-time interface.
> > However, the events that would trigger the need to call memory_failure()
> > over a full PMEM device should be rare. Remove should always be
> > coordinated by the administrator with the filesystem. If someone force
> > removes a device from underneath a mounted filesystem the driver assumes
> > they have a good reason, or otherwise get to keep the pieces. Since
> > ->remove() callbacks can not fail the only option is to trigger the mass
> > memory_failure().
> >
> > The mechanism to determine whether memory_failure() triggers at
> > pmem->remove() time is whether the associated dax_device has an elevated
> > reference at @pgmap ->kill() time.
> >
> > With this in place the get_user_pages_fast() path can drop its
> > half-measure synchronization with an @pgmap reference.
> >
> > Link: http://lore.kernel.org/r/20210224010017.GQ2643399@ziepe.ca [1]
> > Link: http://lore.kernel.org/r/20210302075736.GJ4662@dread.disaster.area [2]
> > Reported-by: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Dave Chinner <david@fromorbit.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> > Cc: Vishal Verma <vishal.l.verma@intel.com>
> > Cc: Dave Jiang <dave.jiang@intel.com>
> > Cc: Ira Weiny <ira.weiny@intel.com>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Jan Kara <jack@suse.cz>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > Cc: "Darrick J. Wong" <djwong@kernel.org>
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > ---
> >  drivers/dax/super.c      |   15 +++++++++++++++
> >  drivers/nvdimm/pmem.c    |   10 +++++++++-
> >  drivers/nvdimm/pmem.h    |    1 +
> >  include/linux/dax.h      |    5 +++++
> >  include/linux/memremap.h |    5 +++++
> >  include/linux/mm.h       |    3 +++
> >  mm/memory-failure.c      |   11 +++++++++--
> >  mm/memremap.c            |   11 +++++++++++
> >  8 files changed, 58 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> > index 5fa6ae9dbc8b..5ebcedf4a68c 100644
> > --- a/drivers/dax/super.c
> > +++ b/drivers/dax/super.c
> > @@ -624,6 +624,21 @@ void put_dax(struct dax_device *dax_dev)
> >  }
> >  EXPORT_SYMBOL_GPL(put_dax);
> >
> > +bool dax_is_idle(struct dax_device *dax_dev)
> > +{
> > +     struct inode *inode;
> > +
> > +     if (!dax_dev)
> > +             return true;
> > +
> > +     WARN_ONCE(test_bit(DAXDEV_ALIVE, &dax_dev->flags),
> > +               "dax idle check on live device.\n");
> > +
> > +     inode = &dax_dev->inode;
> > +     return atomic_read(&inode->i_count) < 2;
> > +}
> > +EXPORT_SYMBOL_GPL(dax_is_idle);
> > +
> >  /**
> >   * dax_get_by_host() - temporary lookup mechanism for filesystem-dax
> >   * @host: alternate name for the device registered by a dax driver
> > diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> > index b8a85bfb2e95..e8822c9262ee 100644
> > --- a/drivers/nvdimm/pmem.c
> > +++ b/drivers/nvdimm/pmem.c
> > @@ -348,15 +348,21 @@ static void pmem_pagemap_kill(struct dev_pagemap *pgmap)
> >  {
> >       struct request_queue *q =
> >               container_of(pgmap->ref, struct request_queue, q_usage_counter);
> > +     struct pmem_device *pmem = q->queuedata;
> >
> >       blk_freeze_queue_start(q);
> > +     kill_dax(pmem->dax_dev);
> > +     if (!dax_is_idle(pmem->dax_dev)) {
> > +             dev_warn(pmem->dev,
> > +                      "DAX active at remove, trigger mass memory failure\n");
> > +             dev_pagemap_failure(pgmap);
> > +     }
> >  }
> >
> >  static void pmem_release_disk(void *__pmem)
> >  {
> >       struct pmem_device *pmem = __pmem;
> >
> > -     kill_dax(pmem->dax_dev);
> >       put_dax(pmem->dax_dev);
> >       del_gendisk(pmem->disk);
> >       put_disk(pmem->disk);
> > @@ -406,6 +412,7 @@ static int pmem_attach_disk(struct device *dev,
> >       devm_namespace_disable(dev, ndns);
> >
> >       dev_set_drvdata(dev, pmem);
> > +     pmem->dev = dev;
> >       pmem->phys_addr = res->start;
> >       pmem->size = resource_size(res);
> >       fua = nvdimm_has_flush(nd_region);
> > @@ -467,6 +474,7 @@ static int pmem_attach_disk(struct device *dev,
> >       blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
> >       if (pmem->pfn_flags & PFN_MAP)
> >               blk_queue_flag_set(QUEUE_FLAG_DAX, q);
> > +     q->queuedata = pmem;
> >
> >       disk = alloc_disk_node(0, nid);
> >       if (!disk)
> > diff --git a/drivers/nvdimm/pmem.h b/drivers/nvdimm/pmem.h
> > index 59cfe13ea8a8..1222088a569a 100644
> > --- a/drivers/nvdimm/pmem.h
> > +++ b/drivers/nvdimm/pmem.h
> > @@ -23,6 +23,7 @@ struct pmem_device {
> >       struct badblocks        bb;
> >       struct dax_device       *dax_dev;
> >       struct gendisk          *disk;
> > +     struct device           *dev;
> >       struct dev_pagemap      pgmap;
> >  };
> >
> > diff --git a/include/linux/dax.h b/include/linux/dax.h
> > index b52f084aa643..015f1d9a8232 100644
> > --- a/include/linux/dax.h
> > +++ b/include/linux/dax.h
> > @@ -46,6 +46,7 @@ struct dax_device *alloc_dax(void *private, const char *host,
> >               const struct dax_operations *ops, unsigned long flags);
> >  void put_dax(struct dax_device *dax_dev);
> >  void kill_dax(struct dax_device *dax_dev);
> > +bool dax_is_idle(struct dax_device *dax_dev);
> >  void dax_write_cache(struct dax_device *dax_dev, bool wc);
> >  bool dax_write_cache_enabled(struct dax_device *dax_dev);
> >  bool __dax_synchronous(struct dax_device *dax_dev);
> > @@ -92,6 +93,10 @@ static inline void put_dax(struct dax_device *dax_dev)
> >  static inline void kill_dax(struct dax_device *dax_dev)
> >  {
> >  }
> > +static inline bool dax_is_idle(struct dax_device *dax_dev)
> > +{
> > +     return true;
> > +}
> >  static inline void dax_write_cache(struct dax_device *dax_dev, bool wc)
> >  {
> >  }
> > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> > index f5b464daeeca..d52cdc6c5313 100644
> > --- a/include/linux/memremap.h
> > +++ b/include/linux/memremap.h
> > @@ -137,6 +137,7 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
> >  void devm_memunmap_pages(struct device *dev, struct dev_pagemap *pgmap);
> >  struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
> >               struct dev_pagemap *pgmap);
> > +void dev_pagemap_failure(struct dev_pagemap *pgmap);
> >  bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);
> >
> >  unsigned long vmem_altmap_offset(struct vmem_altmap *altmap);
> > @@ -160,6 +161,10 @@ static inline void devm_memunmap_pages(struct device *dev,
> >  {
> >  }
> >
> > +static inline void dev_pagemap_failure(struct dev_pagemap *pgmap)
> > +{
> > +}
> > +
> >  static inline struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
> >               struct dev_pagemap *pgmap)
> >  {
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 77e64e3eac80..95f79f457bab 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3002,8 +3002,11 @@ enum mf_flags {
> >       MF_ACTION_REQUIRED = 1 << 1,
> >       MF_MUST_KILL = 1 << 2,
> >       MF_SOFT_OFFLINE = 1 << 3,
> > +     MF_MEM_REMOVE = 1 << 4,
> >  };
> >  extern int memory_failure(unsigned long pfn, int flags);
> > +extern int memory_failure_dev_pagemap(unsigned long pfn, int flags,
> > +                                   struct dev_pagemap *pgmap);
> >  extern void memory_failure_queue(unsigned long pfn, int flags);
> >  extern void memory_failure_queue_kick(int cpu);
> >  extern int unpoison_memory(unsigned long pfn);
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index 43ba4307c526..8f557beb19ee 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -1296,8 +1296,8 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
> >       return res;
> >  }
> >
> > -static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
> > -             struct dev_pagemap *pgmap)
> > +int memory_failure_dev_pagemap(unsigned long pfn, int flags,
> > +                            struct dev_pagemap *pgmap)
> >  {
> >       struct page *page = pfn_to_page(pfn);
> >       const bool unmap_success = true;
> > @@ -1377,6 +1377,13 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
> >  unlock:
> >       dax_unlock_page(page, cookie);
> >  out:
> > +     /*
> > +      * In the removal case, given unmap is always successful, and
> > +      * the driver is responsible for the direct map the recovery is
> > +      * always successful
> > +      */
> > +     if (flags & MF_MEM_REMOVE)
> > +             rc = 0;
> >       action_result(pfn, MF_MSG_DAX, rc ? MF_FAILED : MF_RECOVERED);
> >       return rc;
> >  }
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index 7aa7d6e80ee5..f34da1e14b52 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -165,6 +165,17 @@ static void pageunmap_range(struct dev_pagemap *pgmap, int range_id)
> >       pgmap_array_delete(range);
> >  }
> >
> > +void dev_pagemap_failure(struct dev_pagemap *pgmap)
> > +{
> > +     unsigned long pfn;
> > +     int i;
> > +
> > +     for (i = 0; i < pgmap->nr_range; i++)
> > +             for_each_device_pfn(pfn, pgmap, i)
> > +                     memory_failure_dev_pagemap(pfn, MF_MEM_REMOVE, pgmap);
>
> So my 6TB memory chassis falls off the desk and we have to call
> memory_failure_dev_pagemap for 1.6 billion PFNs?
>

In some respects it's a "doctor it hurts when I remove my devices
without umounting the filesytem first" type situation.

> Honestly if you're going offline the /entire/ device then just tell us
> sb->memory_failure(dev, 0, -1ULL) and we'll just kill everything all at
> once.  That was where I was trying to push Shiyang's patchset, and I had
> nearly succeeded when you NAKd the whole thing.

I think we're closer. As I said to Dave in the other thread what if we
just flipped this around to allow the FS to takeover memory_failure()
rather than hoping that the device-driver can do the right up-calls at
the right time.

>
> In the meantime, I estimate that there are ~45 months worth of deferred
> XFS patch review that I can make progress on, so that's where I'm going
> to focus.

I do feel bad for not engaging sooner on this. Usually there's a
forcing function like Plumbers, or LSF to clear out the backlog.
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