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

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