Re: [v8, 11/18] mm, dax: enable filesystems to trigger dev_pagemap ->page_free callbacks

[Andrei Vagin <avagin@virtuozzo.com>]

Hi Dan,

I catch the following bug on the linux-next 20180404. git bisect brought me to this commit:

commit 8e4d1ccc5286d2c3da6515b92323a3529aa64496 (HEAD, refs/bisect/bad)
Author: Dan Williams <dan.j.williams@intel.com>
Date:   Sat Oct 21 14:41:13 2017 -0700

    mm, dax: enable filesystems to trigger dev_pagemap ->page_free callbacks


[   11.278768] BUG: unable to handle kernel NULL pointer dereference at 0000000000000440
[   11.279999] IP: fs_dax_release+0x5/0x90
[   11.280587] PGD 0 P4D 0 
[   11.280973] Oops: 0000 [#1] SMP PTI
[   11.281500] Modules linked in:
[   11.281968] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 4.16.0-rc4-00193-g8e4d1ccc5286 #7
[   11.283163] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1.fc26 04/01/2014
[   11.284418] RIP: 0010:fs_dax_release+0x5/0x90
[   11.285068] RSP: 0000:ffffb1480062fbd8 EFLAGS: 00010287
[   11.285845] RAX: 0000000000000001 RBX: ffff9e2cb823c088 RCX: 0000000000000003
[   11.286896] RDX: 0000000000000000 RSI: ffff9e2cb823c088 RDI: 0000000000000000
[   11.287980] RBP: ffffb1480062fcd8 R08: 0000000000000001 R09: 0000000000000000
[   11.289147] R10: ffffb1480062fb20 R11: 0000000000000000 R12: 00000000ffffffea
[   11.290576] R13: 0000000000000000 R14: 0000000000000000 R15: ffff9e2cb823a048
[   11.291630] FS:  0000000000000000(0000) GS:ffff9e2cbfd00000(0000) knlGS:0000000000000000
[   11.292781] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   11.293602] CR2: 0000000000000440 CR3: 000000007d21e001 CR4: 00000000003606e0
[   11.294817] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   11.296827] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   11.298293] Call Trace:
[   11.298728]  ext4_fill_super+0x31b/0x39d0
[   11.299441]  ? sget_userns+0x155/0x500
[   11.300144]  ? vsnprintf+0x253/0x4b0
[   11.301223]  ? ext4_calculate_overhead+0x4a0/0x4a0
[   11.301801]  ? snprintf+0x45/0x70
[   11.302214]  ? ext4_calculate_overhead+0x4a0/0x4a0
[   11.302822]  mount_bdev+0x17b/0x1b0
[   11.303332]  mount_fs+0x35/0x150
[   11.303803]  vfs_kern_mount.part.25+0x54/0x150
[   11.304443]  do_mount+0x620/0xd60
[   11.304935]  ? memdup_user+0x3e/0x70
[   11.305458]  SyS_mount+0x80/0xd0
[   11.305931]  mount_block_root+0x105/0x2b7
[   11.306512]  ? SyS_mknod+0x16b/0x1f0
[   11.307035]  ? set_debug_rodata+0x11/0x11
[   11.307616]  prepare_namespace+0x135/0x16b
[   11.308215]  kernel_init_freeable+0x271/0x297
[   11.308838]  ? rest_init+0xd0/0xd0
[   11.309322]  kernel_init+0xa/0x110
[   11.309821]  ret_from_fork+0x3a/0x50
[   11.310347] Code: a5 45 31 ed e8 5d 5e 36 00 eb d7 48 c7 c7 20 48 2f a5 e8 4f 5e 36 00 eb c9 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 <48> 8b 87 40 04 00 00 48 8b 40 18 48 85 c0 74 05 e9 c6 7e 60 00 
[   11.313168] RIP: fs_dax_release+0x5/0x90 RSP: ffffb1480062fbd8
[   11.313991] CR2: 0000000000000440
[   11.314475] ---[ end trace 8acbb19b74409665 ]---


On Fri, Mar 30, 2018 at 09:03:08PM -0700, Dan Williams 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>
> ---
>  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   |   23 ++++++++++++++---------
>  6 files changed, 43 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index c4cf284dfe1c..7d260f118a39 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
> @@ -579,6 +569,17 @@ 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 06f8dcc52ca6..e6d7351f3379 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 7666c065b96f..6ae20e319bc4 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 39bf464c35f1..315a323729e3 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -952,7 +952,7 @@ 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);
>  }
>  
> @@ -3398,7 +3398,7 @@ 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;
> @@ -4408,7 +4408,7 @@ 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 93588ea3d3d2..ef7dd7148c0b 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -724,7 +724,7 @@ xfs_close_devices(
>  
>  		xfs_free_buftarg(mp, 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, mp->m_rtdev_targp);
>  		xfs_blkdev_put(rtdev);
> -		fs_put_dax(dax_rtdev);
> +		fs_dax_release(dax_rtdev, mp);
>  	}
>  	xfs_free_buftarg(mp, 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, 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 e9d59a6b06e1..a88ff009e2a1 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -32,6 +32,8 @@ 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);
> @@ -50,6 +52,12 @@ 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)
>  {
>  }
> @@ -79,12 +87,8 @@ static inline struct dax_device *fs_dax_get_by_host(const char *host)
>  	return dax_get_by_host(host);
>  }
>  
> -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);
> +struct dax_device *fs_dax_claim_bdev(struct block_device *bdev, void *owner);
> +void fs_dax_release(struct dax_device *dax_dev, void *owner);
>  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);
> @@ -100,13 +104,14 @@ static inline struct dax_device *fs_dax_get_by_host(const char *host)
>  	return NULL;
>  }
>  
> -static inline void fs_put_dax(struct dax_device *dax_dev)
> +static inline struct dax_device *fs_dax_claim_bdev(struct block_device *bdev,
> +		void *owner)
>  {
> +	return NULL;
>  }
>  
> -static inline struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev)
> +static inline void fs_dax_release(struct dax_device *dax_dev, void *owner)
>  {
> -	return NULL;
>  }
>  
>  static inline int dax_writeback_mapping_range(struct address_space *mapping,