Commit 2dfb1e43 authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba
Browse files

btrfs: preallocate anon block device at first phase of snapshot creation 



[BUG]
When the anonymous block device pool is exhausted, subvolume/snapshot
creation fails with EMFILE (Too many files open). This has been reported
by a user. The allocation happens in the second phase during transaction
commit where it's only way out is to abort the transaction

  BTRFS: Transaction aborted (error -24)
  WARNING: CPU: 17 PID: 17041 at fs/btrfs/transaction.c:1576 create_pending_snapshot+0xbc4/0xd10 [btrfs]
  RIP: 0010:create_pending_snapshot+0xbc4/0xd10 [btrfs]
  Call Trace:
   create_pending_snapshots+0x82/0xa0 [btrfs]
   btrfs_commit_transaction+0x275/0x8c0 [btrfs]
   btrfs_mksubvol+0x4b9/0x500 [btrfs]
   btrfs_ioctl_snap_create_transid+0x174/0x180 [btrfs]
   btrfs_ioctl_snap_create_v2+0x11c/0x180 [btrfs]
   btrfs_ioctl+0x11a4/0x2da0 [btrfs]
   do_vfs_ioctl+0xa9/0x640
   ksys_ioctl+0x67/0x90
   __x64_sys_ioctl+0x1a/0x20
   do_syscall_64+0x5a/0x110
   entry_SYSCALL_64_after_hwframe+0x44/0xa9
  ---[ end trace 33f2f83f3d5250e9 ]---
  BTRFS: error (device sda1) in create_pending_snapshot:1576: errno=-24 unknown
  BTRFS info (device sda1): forced readonly
  BTRFS warning (device sda1): Skipping commit of aborted transaction.
  BTRFS: error (device sda1) in cleanup_transaction:1831: errno=-24 unknown

[CAUSE]
When the global anonymous block device pool is exhausted, the following
call chain will fail, and lead to transaction abort:

 btrfs_ioctl_snap_create_v2()
 |- btrfs_ioctl_snap_create_transid()
    |- btrfs_mksubvol()
       |- btrfs_commit_transaction()
          |- create_pending_snapshot()
             |- btrfs_get_fs_root()
                |- btrfs_init_fs_root()
                   |- get_anon_bdev()

[FIX]
Although we can't enlarge the anonymous block device pool, at least we
can preallocate anon_dev for subvolume/snapshot in the first phase,
outside of transaction context and exactly at the moment the user calls
the creation ioctl.

Reported-by: default avatarGreed Rong <greedrong@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/CA+UqX+NTrZ6boGnWHhSeZmEY5J76CTqmYjO2S+=tHJX7nb9DPw@mail.gmail.com/


CC: stable@vger.kernel.org # 4.4+
Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 082b6c97

fs/btrfs/disk-io.c

+64 −7
Original line number Diff line number Diff line
@@ -1391,7 +1391,12 @@ alloc_fail: 
	goto out;

}



static int btrfs_init_fs_root(struct btrfs_root *root)

/*

 * Initialize subvolume root in-memory structure

 *

 * @anon_dev:	anonymous device to attach to the root, if zero, allocate new

 */

static int btrfs_init_fs_root(struct btrfs_root *root, dev_t anon_dev)

{

	int ret;

	unsigned int nofs_flag;
@@ -1430,9 +1435,13 @@ static int btrfs_init_fs_root(struct btrfs_root *root) 
	 */

	if (is_fstree(root->root_key.objectid) &&

	    btrfs_root_refs(&root->root_item) > 0) {

		if (!anon_dev) {

			ret = get_anon_bdev(&root->anon_dev);

			if (ret)

				goto fail;

		} else {

			root->anon_dev = anon_dev;

		}

	}



	mutex_lock(&root->objectid_mutex);
@@ -1537,8 +1546,27 @@ void btrfs_free_fs_info(struct btrfs_fs_info *fs_info) 
}





struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info,

				     u64 objectid, bool check_ref)

/*

 * Get an in-memory reference of a root structure.

 *

 * For essential trees like root/extent tree, we grab it from fs_info directly.

 * For subvolume trees, we check the cached filesystem roots first. If not

 * found, then read it from disk and add it to cached fs roots.

 *

 * Caller should release the root by calling btrfs_put_root() after the usage.

 *

 * NOTE: Reloc and log trees can't be read by this function as they share the

 *	 same root objectid.

 *

 * @objectid:	root id

 * @anon_dev:	preallocated anonymous block device number for new roots,

 * 		pass 0 for new allocation.

 * @check_ref:	whether to check root item references, If true, return -ENOENT

 *		for orphan roots

 */

static struct btrfs_root *btrfs_get_root_ref(struct btrfs_fs_info *fs_info,

					     u64 objectid, dev_t anon_dev,

					     bool check_ref)

{

	struct btrfs_root *root;

	struct btrfs_path *path;
@@ -1567,6 +1595,8 @@ struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info, 
again:

	root = btrfs_lookup_fs_root(fs_info, objectid);

	if (root) {

		/* Shouldn't get preallocated anon_dev for cached roots */

		ASSERT(!anon_dev);

		if (check_ref && btrfs_root_refs(&root->root_item) == 0) {

			btrfs_put_root(root);

			return ERR_PTR(-ENOENT);
@@ -1586,7 +1616,7 @@ again: 
		goto fail;

	}



	ret = btrfs_init_fs_root(root);

	ret = btrfs_init_fs_root(root, anon_dev);

	if (ret)

		goto fail;
@@ -1619,6 +1649,33 @@ fail: 
	return ERR_PTR(ret);

}



/*

 * Get in-memory reference of a root structure

 *

 * @objectid:	tree objectid

 * @check_ref:	if set, verify that the tree exists and the item has at least

 *		one reference

 */

struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info,

				     u64 objectid, bool check_ref)

{

	return btrfs_get_root_ref(fs_info, objectid, 0, check_ref);

}



/*

 * Get in-memory reference of a root structure, created as new, optionally pass

 * the anonymous block device id

 *

 * @objectid:	tree objectid

 * @anon_dev:	if zero, allocate a new anonymous block device or use the

 *		parameter value

 */

struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info,

					 u64 objectid, dev_t anon_dev)

{

	return btrfs_get_root_ref(fs_info, objectid, anon_dev, true);

}



static int btrfs_congested_fn(void *congested_data, int bdi_bits)

{

	struct btrfs_fs_info *info = (struct btrfs_fs_info *)congested_data;

fs/btrfs/disk-io.h

+2 −0
Original line number Diff line number Diff line
@@ -67,6 +67,8 @@ void btrfs_free_fs_roots(struct btrfs_fs_info *fs_info); 


struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info,

				     u64 objectid, bool check_ref);

struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info,

					 u64 objectid, dev_t anon_dev);



void btrfs_free_fs_info(struct btrfs_fs_info *fs_info);

int btrfs_cleanup_fs_roots(struct btrfs_fs_info *fs_info);

fs/btrfs/ioctl.c

+20 −1
Original line number Diff line number Diff line
@@ -566,6 +566,7 @@ static noinline int create_subvol(struct inode *dir, 
	struct inode *inode;

	int ret;

	int err;

	dev_t anon_dev = 0;

	u64 objectid;

	u64 new_dirid = BTRFS_FIRST_FREE_OBJECTID;

	u64 index = 0;
@@ -578,6 +579,10 @@ static noinline int create_subvol(struct inode *dir, 
	if (ret)

		goto fail_free;



	ret = get_anon_bdev(&anon_dev);

	if (ret < 0)

		goto fail_free;



	/*

	 * Don't create subvolume whose level is not zero. Or qgroup will be

	 * screwed up since it assumes subvolume qgroup's level to be 0.
@@ -660,12 +665,15 @@ static noinline int create_subvol(struct inode *dir, 
		goto fail;



	key.offset = (u64)-1;

	new_root = btrfs_get_fs_root(fs_info, objectid, true);

	new_root = btrfs_get_new_fs_root(fs_info, objectid, anon_dev);

	if (IS_ERR(new_root)) {

		free_anon_bdev(anon_dev);

		ret = PTR_ERR(new_root);

		btrfs_abort_transaction(trans, ret);

		goto fail;

	}

	/* Freeing will be done in btrfs_put_root() of new_root */

	anon_dev = 0;



	btrfs_record_root_in_trans(trans, new_root);
@@ -735,6 +743,8 @@ fail: 
	return ret;



fail_free:

	if (anon_dev)

		free_anon_bdev(anon_dev);

	kfree(root_item);

	return ret;

}
@@ -762,6 +772,9 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir, 
	if (!pending_snapshot)

		return -ENOMEM;



	ret = get_anon_bdev(&pending_snapshot->anon_dev);

	if (ret < 0)

		goto free_pending;

	pending_snapshot->root_item = kzalloc(sizeof(struct btrfs_root_item),

			GFP_KERNEL);

	pending_snapshot->path = btrfs_alloc_path();
@@ -823,10 +836,16 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir, 


	d_instantiate(dentry, inode);

	ret = 0;

	pending_snapshot->anon_dev = 0;

fail:

	/* Prevent double freeing of anon_dev */

	if (ret && pending_snapshot->snap)

		pending_snapshot->snap->anon_dev = 0;

	btrfs_put_root(pending_snapshot->snap);

	btrfs_subvolume_release_metadata(fs_info, &pending_snapshot->block_rsv);

free_pending:

	if (pending_snapshot->anon_dev)

		free_anon_bdev(pending_snapshot->anon_dev);

	kfree(pending_snapshot->root_item);

	btrfs_free_path(pending_snapshot->path);

	kfree(pending_snapshot);

fs/btrfs/transaction.c

+1 −1
Original line number Diff line number Diff line
@@ -1630,7 +1630,7 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans, 
	}



	key.offset = (u64)-1;

	pending->snap = btrfs_get_fs_root(fs_info, objectid, true);

	pending->snap = btrfs_get_new_fs_root(fs_info, objectid, pending->anon_dev);

	if (IS_ERR(pending->snap)) {

		ret = PTR_ERR(pending->snap);

		btrfs_abort_transaction(trans, ret);

fs/btrfs/transaction.h

+2 −0
Original line number Diff line number Diff line
@@ -151,6 +151,8 @@ struct btrfs_pending_snapshot { 
	struct btrfs_block_rsv block_rsv;

	/* extra metadata reservation for relocation */

	int error;

	/* Preallocated anonymous block device number */

	dev_t anon_dev;

	bool readonly;

	struct list_head list;

};

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