Re: [PATCH v8 10/18] dax, dm: introduce ->fs_{claim, release}() dax_device infrastructure

[Dan Williams]

On Fri, Mar 30, 2018 at 9:03 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> In preparation for allowing filesystems to augment the dev_pagemap
> associated with a dax_device, add an ->fs_claim() callback. The
> ->fs_claim() callback is leveraged by the device-mapper dax
> implementation to iterate all member devices in the map and repeat the
> claim operation across the array.
>
> In order to resolve collisions between filesystem operations and DMA to
> DAX mapped pages we need a callback when DMA completes. With a callback
> we can hold off filesystem operations while DMA is in-flight and then
> resume those operations when the last put_page() occurs on a DMA page.
> The ->fs_claim() operation arranges for this callback to be registered,
> although that implementation is saved for a later patch.
>
> Cc: Alasdair Kergon <agk@xxxxxxxxxx>
> Cc: Mike Snitzer <snitzer@xxxxxxxxxx>

Mike, do these DM touches look ok to you?  We need these ->fs_claim()
/ ->fs_release() interfaces for device-mapper to set up filesystem-dax
infrastructure on all sub-devices whenever a dax-capable DM device is
mounted. It builds on the device-mapper dax dependency removal
patches.

> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  drivers/dax/super.c      |   80 ++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/md/dm.c          |   56 ++++++++++++++++++++++++++++++++
>  include/linux/dax.h      |   16 +++++++++
>  include/linux/memremap.h |    8 +++++
>  4 files changed, 160 insertions(+)
>
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index 2b2332b605e4..c4cf284dfe1c 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -29,6 +29,7 @@ static struct vfsmount *dax_mnt;
>  static DEFINE_IDA(dax_minor_ida);
>  static struct kmem_cache *dax_cache __read_mostly;
>  static struct super_block *dax_superblock __read_mostly;
> +static DEFINE_MUTEX(devmap_lock);
>
>  #define DAX_HASH_SIZE (PAGE_SIZE / sizeof(struct hlist_head))
>  static struct hlist_head dax_host_list[DAX_HASH_SIZE];
> @@ -169,9 +170,88 @@ struct dax_device {
>         const char *host;
>         void *private;
>         unsigned long flags;
> +       struct dev_pagemap *pgmap;
>         const struct dax_operations *ops;
>  };
>
> +#if IS_ENABLED(CONFIG_FS_DAX)
> +static void generic_dax_pagefree(struct page *page, void *data)
> +{
> +       /* TODO: wakeup page-idle waiters */
> +}
> +
> +struct dax_device *fs_dax_claim(struct dax_device *dax_dev, void *owner)
> +{
> +       struct dev_pagemap *pgmap;
> +
> +       if (!dax_dev->pgmap)
> +               return dax_dev;
> +       pgmap = dax_dev->pgmap;
> +
> +       mutex_lock(&devmap_lock);
> +       if (pgmap->data && pgmap->data == owner) {
> +               /* dm might try to claim the same device more than once... */
> +               mutex_unlock(&devmap_lock);
> +               return dax_dev;
> +       } else if (pgmap->page_free || pgmap->page_fault
> +                       || pgmap->type != MEMORY_DEVICE_HOST) {
> +               put_dax(dax_dev);
> +               mutex_unlock(&devmap_lock);
> +               return NULL;
> +       }
> +
> +       pgmap->type = MEMORY_DEVICE_FS_DAX;
> +       pgmap->page_free = generic_dax_pagefree;
> +       pgmap->data = owner;
> +       mutex_unlock(&devmap_lock);
> +
> +       return dax_dev;
> +}
> +EXPORT_SYMBOL_GPL(fs_dax_claim);
> +
> +struct dax_device *fs_dax_claim_bdev(struct block_device *bdev, void *owner)
> +{
> +       struct dax_device *dax_dev;
> +
> +       if (!blk_queue_dax(bdev->bd_queue))
> +               return NULL;
> +       dax_dev = fs_dax_get_by_host(bdev->bd_disk->disk_name);
> +       if (dax_dev->ops->fs_claim)
> +               return dax_dev->ops->fs_claim(dax_dev, owner);
> +       else
> +               return fs_dax_claim(dax_dev, owner);
> +}
> +EXPORT_SYMBOL_GPL(fs_dax_claim_bdev);
> +
> +void __fs_dax_release(struct dax_device *dax_dev, void *owner)
> +{
> +       struct dev_pagemap *pgmap = dax_dev ? dax_dev->pgmap : NULL;
> +
> +       put_dax(dax_dev);
> +       if (!pgmap)
> +               return;
> +       if (!pgmap->data)
> +               return;
> +
> +       mutex_lock(&devmap_lock);
> +       WARN_ON(pgmap->data != owner);
> +       pgmap->type = MEMORY_DEVICE_HOST;
> +       pgmap->page_free = NULL;
> +       pgmap->data = NULL;
> +       mutex_unlock(&devmap_lock);
> +}
> +EXPORT_SYMBOL_GPL(__fs_dax_release);
> +
> +void fs_dax_release(struct dax_device *dax_dev, void *owner)
> +{
> +       if (dax_dev->ops->fs_release)
> +               dax_dev->ops->fs_release(dax_dev, owner);
> +       else
> +               __fs_dax_release(dax_dev, owner);
> +}
> +EXPORT_SYMBOL_GPL(fs_dax_release);
> +#endif
> +
>  static ssize_t write_cache_show(struct device *dev,
>                 struct device_attribute *attr, char *buf)
>  {
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index ffc93aecc02a..964cb7537f11 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1090,6 +1090,60 @@ static size_t dm_dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff,
>         return ret;
>  }
>
> +static int dm_dax_dev_claim(struct dm_target *ti, struct dm_dev *dev,
> +               sector_t start, sector_t len, void *owner)
> +{
> +       if (fs_dax_claim(dev->dax_dev, owner))
> +               return 0;
> +       /*
> +        * Outside of a kernel bug there is no reason a dax_dev should
> +        * fail a claim attempt. Device-mapper should have exclusive
> +        * ownership of the dm_dev and the filesystem should have
> +        * exclusive ownership of the dm_target.
> +        */
> +       WARN_ON_ONCE(1);
> +       return -ENXIO;
> +}
> +
> +static int dm_dax_dev_release(struct dm_target *ti, struct dm_dev *dev,
> +               sector_t start, sector_t len, void *owner)
> +{
> +       __fs_dax_release(dev->dax_dev, owner);
> +       return 0;
> +}
> +
> +static struct dax_device *dm_dax_iterate(struct dax_device *dax_dev,
> +               iterate_devices_callout_fn fn, void *arg)
> +{
> +       struct mapped_device *md = dax_get_private(dax_dev);
> +       struct dm_table *map;
> +       struct dm_target *ti;
> +       int i, srcu_idx;
> +
> +       map = dm_get_live_table(md, &srcu_idx);
> +
> +       for (i = 0; i < dm_table_get_num_targets(map); i++) {
> +               ti = dm_table_get_target(map, i);
> +
> +               if (ti->type->iterate_devices)
> +                       ti->type->iterate_devices(ti, fn, arg);
> +       }
> +
> +       dm_put_live_table(md, srcu_idx);
> +       return dax_dev;
> +}
> +
> +static struct dax_device *dm_dax_fs_claim(struct dax_device *dax_dev,
> +               void *owner)
> +{
> +       return dm_dax_iterate(dax_dev, dm_dax_dev_claim, owner);
> +}
> +
> +static void dm_dax_fs_release(struct dax_device *dax_dev, void *owner)
> +{
> +       dm_dax_iterate(dax_dev, dm_dax_dev_release, owner);
> +}
> +
>  /*
>   * A target may call dm_accept_partial_bio only from the map routine.  It is
>   * allowed for all bio types except REQ_PREFLUSH and REQ_OP_ZONE_RESET.
> @@ -3111,6 +3165,8 @@ static const struct block_device_operations dm_blk_dops = {
>  static const struct dax_operations dm_dax_ops = {
>         .direct_access = dm_dax_direct_access,
>         .copy_from_iter = dm_dax_copy_from_iter,
> +       .fs_claim = dm_dax_fs_claim,
> +       .fs_release = dm_dax_fs_release,
>  };
>
>  /*
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index f9eb22ad341e..e9d59a6b06e1 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -20,6 +20,10 @@ struct dax_operations {
>         /* copy_from_iter: required operation for fs-dax direct-i/o */
>         size_t (*copy_from_iter)(struct dax_device *, pgoff_t, void *, size_t,
>                         struct iov_iter *);
> +       /* fs_claim: setup filesytem parameters for the device's dev_pagemap */
> +       struct dax_device *(*fs_claim)(struct dax_device *, void *);
> +       /* fs_release: restore device's dev_pagemap to its default state */
> +       void (*fs_release)(struct dax_device *, void *);
>  };
>
>  extern struct attribute_group dax_attribute_group;
> @@ -83,6 +87,8 @@ static inline void fs_put_dax(struct dax_device *dax_dev)
>  struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev);
>  int dax_writeback_mapping_range(struct address_space *mapping,
>                 struct block_device *bdev, struct writeback_control *wbc);
> +struct dax_device *fs_dax_claim(struct dax_device *dax_dev, void *owner);
> +void __fs_dax_release(struct dax_device *dax_dev, void *owner);
>  #else
>  static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
>  {
> @@ -108,6 +114,16 @@ static inline int dax_writeback_mapping_range(struct address_space *mapping,
>  {
>         return -EOPNOTSUPP;
>  }
> +
> +static inline struct dax_device *fs_dax_claim(struct dax_device *dax_dev,
> +               void *owner)
> +{
> +       return NULL;
> +}
> +
> +static inline void __fs_dax_release(struct dax_device *dax_dev, void *owner)
> +{
> +}
>  #endif
>
>  int dax_read_lock(void);
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 7b4899c06f49..02d6d042ee7f 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -53,11 +53,19 @@ struct vmem_altmap {
>   * driver can hotplug the device memory using ZONE_DEVICE and with that memory
>   * type. Any page of a process can be migrated to such memory. However no one
>   * should be allow to pin such memory so that it can always be evicted.
> + *
> + * MEMORY_DEVICE_FS_DAX:
> + * When MEMORY_DEVICE_HOST memory is represented by a device that can
> + * host a filesystem, for example /dev/pmem0, that filesystem can
> + * register for a callback when a page is idled. For the filesystem-dax
> + * case page idle callbacks are used to coordinate DMA vs
> + * hole-punch/truncate.
>   */
>  enum memory_type {
>         MEMORY_DEVICE_HOST = 0,
>         MEMORY_DEVICE_PRIVATE,
>         MEMORY_DEVICE_PUBLIC,
> +       MEMORY_DEVICE_FS_DAX,
>  };
>
>  /*
>