Re: [PATCH v9 2/9] mm, dax: enable filesystems to trigger dev_pagemap ->page_free callbacks

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

On Tue, Apr 24, 2018 at 4:33 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> 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.
>
> Recall that the 'struct page' entries for DAX memory are created with
> devm_memremap_pages(). That routine arranges for the pages to be
> allocated, but never onlined, so a DAX page is DMA-idle when its
> reference count reaches one.
>
> Also recall that the HMM sub-system added infrastructure to trap the
> page-idle (2-to-1 reference count) transition of the pages allocated by
> devm_memremap_pages() and trigger a callback via the 'struct
> dev_pagemap' associated with the page range. Whereas the HMM callbacks
> are going to a device driver to manage bounce pages in device-memory in
> the filesystem-dax case we will call back to filesystem specified
> callback.
>
> Since the callback is not known at devm_memremap_pages() time we arrange
> for the filesystem to install it at mount time. No functional changes
> are expected as this only registers a nop handler for the ->page_free()
> event for device-mapped pages.
>
> Cc: Michal Hocko <mhocko@suse.com>
> Reviewed-by: "Jérôme Glisse" <jglisse@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---

Ugh, so it turns out this approach is broken in the face of
partitions. Thanks to Dave J for noticing this with xfstests, I was
only testing with whole devices.

Thankfully, the fix is straightforward. Just delete this and patch1
and move generic_dax_pagefree() into the pmem driver directly. It's a
1:1 relationship between pgmap and the dax_device, so there is no need
for this dynamic claim infrastructure. I'll send out the patches that
got modified after rebasing on the removal of this patch.

>  drivers/dax/super.c   |   21 +++++++++++----------
>  drivers/nvdimm/pmem.c |    3 ++-
>  fs/ext2/super.c       |    6 +++---
>  fs/ext4/super.c       |    6 +++---
>  fs/xfs/xfs_super.c    |   20 ++++++++++----------
>  include/linux/dax.h   |   14 ++++++++------
>  6 files changed, 37 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index e62a64b9c9fb..e4864f319e16 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -63,16 +63,6 @@ int bdev_dax_pgoff(struct block_device *bdev, sector_t sector, size_t size,
>  }
>  EXPORT_SYMBOL(bdev_dax_pgoff);
>
> -#if IS_ENABLED(CONFIG_FS_DAX)
> -struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev)
> -{
> -       if (!blk_queue_dax(bdev->bd_queue))
> -               return NULL;
> -       return fs_dax_get_by_host(bdev->bd_disk->disk_name);
> -}
> -EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
> -#endif
> -
>  /**
>   * __bdev_dax_supported() - Check if the device supports dax for filesystem
>   * @sb: The superblock of the device
> @@ -575,6 +565,17 @@ struct dax_device *alloc_dax(void *private, const char *__host,
>  }
>  EXPORT_SYMBOL_GPL(alloc_dax);
>
> +struct dax_device *alloc_dax_devmap(void *private, const char *host,
> +               const struct dax_operations *ops, struct dev_pagemap *pgmap)
> +{
> +       struct dax_device *dax_dev = alloc_dax(private, host, ops);
> +
> +       if (dax_dev)
> +               dax_dev->pgmap = pgmap;
> +       return dax_dev;
> +}
> +EXPORT_SYMBOL_GPL(alloc_dax_devmap);
> +
>  void put_dax(struct dax_device *dax_dev)
>  {
>         if (!dax_dev)
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 9d714926ecf5..fc1a1ab25e9e 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -408,7 +408,8 @@ static int pmem_attach_disk(struct device *dev,
>         nvdimm_badblocks_populate(nd_region, &pmem->bb, &bb_res);
>         disk->bb = &pmem->bb;
>
> -       dax_dev = alloc_dax(pmem, disk->disk_name, &pmem_dax_ops);
> +       dax_dev = alloc_dax_devmap(pmem, disk->disk_name, &pmem_dax_ops,
> +                       &pmem->pgmap);
>         if (!dax_dev) {
>                 put_disk(disk);
>                 return -ENOMEM;
> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> index de1694512f1f..421c7d4bed39 100644
> --- a/fs/ext2/super.c
> +++ b/fs/ext2/super.c
> @@ -172,7 +172,7 @@ static void ext2_put_super (struct super_block * sb)
>         brelse (sbi->s_sbh);
>         sb->s_fs_info = NULL;
>         kfree(sbi->s_blockgroup_lock);
> -       fs_put_dax(sbi->s_daxdev);
> +       fs_dax_release(sbi->s_daxdev, sb);
>         kfree(sbi);
>  }
>
> @@ -817,7 +817,7 @@ static unsigned long descriptor_loc(struct super_block *sb,
>
>  static int ext2_fill_super(struct super_block *sb, void *data, int silent)
>  {
> -       struct dax_device *dax_dev = fs_dax_get_by_bdev(sb->s_bdev);
> +       struct dax_device *dax_dev = fs_dax_claim_bdev(sb->s_bdev, sb);
>         struct buffer_head * bh;
>         struct ext2_sb_info * sbi;
>         struct ext2_super_block * es;
> @@ -1213,7 +1213,7 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
>         kfree(sbi->s_blockgroup_lock);
>         kfree(sbi);
>  failed:
> -       fs_put_dax(dax_dev);
> +       fs_dax_release(dax_dev, sb);
>         return ret;
>  }
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 185f7e61f4cf..3e5d0f9e8772 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -954,7 +954,7 @@ static void ext4_put_super(struct super_block *sb)
>         if (sbi->s_chksum_driver)
>                 crypto_free_shash(sbi->s_chksum_driver);
>         kfree(sbi->s_blockgroup_lock);
> -       fs_put_dax(sbi->s_daxdev);
> +       fs_dax_release(sbi->s_daxdev, sb);
>         kfree(sbi);
>  }
>
> @@ -3407,7 +3407,7 @@ static void ext4_set_resv_clusters(struct super_block *sb)
>
>  static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>  {
> -       struct dax_device *dax_dev = fs_dax_get_by_bdev(sb->s_bdev);
> +       struct dax_device *dax_dev = fs_dax_claim_bdev(sb->s_bdev, sb);
>         char *orig_data = kstrdup(data, GFP_KERNEL);
>         struct buffer_head *bh;
>         struct ext4_super_block *es = NULL;
> @@ -4429,7 +4429,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>  out_free_base:
>         kfree(sbi);
>         kfree(orig_data);
> -       fs_put_dax(dax_dev);
> +       fs_dax_release(dax_dev, sb);
>         return err ? err : ret;
>  }
>
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index d71424052917..f53f8a47a526 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -724,7 +724,7 @@ xfs_close_devices(
>
>                 xfs_free_buftarg(mp->m_logdev_targp);
>                 xfs_blkdev_put(logdev);
> -               fs_put_dax(dax_logdev);
> +               fs_dax_release(dax_logdev, mp);
>         }
>         if (mp->m_rtdev_targp) {
>                 struct block_device *rtdev = mp->m_rtdev_targp->bt_bdev;
> @@ -732,10 +732,10 @@ xfs_close_devices(
>
>                 xfs_free_buftarg(mp->m_rtdev_targp);
>                 xfs_blkdev_put(rtdev);
> -               fs_put_dax(dax_rtdev);
> +               fs_dax_release(dax_rtdev, mp);
>         }
>         xfs_free_buftarg(mp->m_ddev_targp);
> -       fs_put_dax(dax_ddev);
> +       fs_dax_release(dax_ddev, mp);
>  }
>
>  /*
> @@ -753,9 +753,9 @@ xfs_open_devices(
>         struct xfs_mount        *mp)
>  {
>         struct block_device     *ddev = mp->m_super->s_bdev;
> -       struct dax_device       *dax_ddev = fs_dax_get_by_bdev(ddev);
> -       struct dax_device       *dax_logdev = NULL, *dax_rtdev = NULL;
> +       struct dax_device       *dax_ddev = fs_dax_claim_bdev(ddev, mp);
>         struct block_device     *logdev = NULL, *rtdev = NULL;
> +       struct dax_device       *dax_logdev = NULL, *dax_rtdev = NULL;
>         int                     error;
>
>         /*
> @@ -765,7 +765,7 @@ xfs_open_devices(
>                 error = xfs_blkdev_get(mp, mp->m_logname, &logdev);
>                 if (error)
>                         goto out;
> -               dax_logdev = fs_dax_get_by_bdev(logdev);
> +               dax_logdev = fs_dax_claim_bdev(logdev, mp);
>         }
>
>         if (mp->m_rtname) {
> @@ -779,7 +779,7 @@ xfs_open_devices(
>                         error = -EINVAL;
>                         goto out_close_rtdev;
>                 }
> -               dax_rtdev = fs_dax_get_by_bdev(rtdev);
> +               dax_rtdev = fs_dax_claim_bdev(rtdev, mp);
>         }
>
>         /*
> @@ -813,14 +813,14 @@ xfs_open_devices(
>         xfs_free_buftarg(mp->m_ddev_targp);
>   out_close_rtdev:
>         xfs_blkdev_put(rtdev);
> -       fs_put_dax(dax_rtdev);
> +       fs_dax_release(dax_rtdev, mp);
>   out_close_logdev:
>         if (logdev && logdev != ddev) {
>                 xfs_blkdev_put(logdev);
> -               fs_put_dax(dax_logdev);
> +               fs_dax_release(dax_logdev, mp);
>         }
>   out:
> -       fs_put_dax(dax_ddev);
> +       fs_dax_release(dax_ddev, mp);
>         return error;
>  }
>
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index af02f93c943a..fe322d67856e 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -33,6 +33,8 @@ extern struct attribute_group dax_attribute_group;
>  struct dax_device *dax_get_by_host(const char *host);
>  struct dax_device *alloc_dax(void *private, const char *host,
>                 const struct dax_operations *ops);
> +struct dax_device *alloc_dax_devmap(void *private, const char *host,
> +               const struct dax_operations *ops, struct dev_pagemap *pgmap);
>  void put_dax(struct dax_device *dax_dev);
>  void kill_dax(struct dax_device *dax_dev);
>  void dax_write_cache(struct dax_device *dax_dev, bool wc);
> @@ -51,6 +53,12 @@ static inline struct dax_device *alloc_dax(void *private, const char *host,
>          */
>         return NULL;
>  }
> +static inline struct dax_device *alloc_dax_devmap(void *private,
> +               const char *host, const struct dax_operations *ops,
> +               struct dev_pagemap *pgmap)
> +{
> +       return NULL;
> +}
>  static inline void put_dax(struct dax_device *dax_dev)
>  {
>  }
> @@ -85,7 +93,6 @@ static inline void fs_put_dax(struct dax_device *dax_dev)
>         put_dax(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);
> @@ -123,11 +130,6 @@ static inline void fs_put_dax(struct dax_device *dax_dev)
>  {
>  }
>
> -static inline struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev)
> -{
> -       return NULL;
> -}
> -
>  static inline int dax_writeback_mapping_range(struct address_space *mapping,
>                 struct block_device *bdev, struct writeback_control *wbc)
>  {
>
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