From: Filipe Manana Date: Tue, 16 Dec 2025 14:51:52 +0000 (+0000) Subject: btrfs: release path before initializing extent tree in btrfs_read_locked_inode() X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8731f2c50b0b1d2b58ed5b9671ef2c4bdc2f8347;p=thirdparty%2Fkernel%2Flinux.git btrfs: release path before initializing extent tree in btrfs_read_locked_inode() In btrfs_read_locked_inode() we are calling btrfs_init_file_extent_tree() while holding a path with a read locked leaf from a subvolume tree, and btrfs_init_file_extent_tree() may do a GFP_KERNEL allocation, which can trigger reclaim. This can create a circular lock dependency which lockdep warns about with the following splat: [6.1433] ====================================================== [6.1574] WARNING: possible circular locking dependency detected [6.1583] 6.18.0+ #4 Tainted: G U [6.1591] ------------------------------------------------------ [6.1599] kswapd0/117 is trying to acquire lock: [6.1606] ffff8d9b6333c5b8 (&delayed_node->mutex){+.+.}-{3:3}, at: __btrfs_release_delayed_node.part.0+0x39/0x2f0 [6.1625] but task is already holding lock: [6.1633] ffffffffa4ab8ce0 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat+0x195/0xc60 [6.1646] which lock already depends on the new lock. [6.1657] the existing dependency chain (in reverse order) is: [6.1667] -> #2 (fs_reclaim){+.+.}-{0:0}: [6.1677] fs_reclaim_acquire+0x9d/0xd0 [6.1685] __kmalloc_cache_noprof+0x59/0x750 [6.1694] btrfs_init_file_extent_tree+0x90/0x100 [6.1702] btrfs_read_locked_inode+0xc3/0x6b0 [6.1710] btrfs_iget+0xbb/0xf0 [6.1716] btrfs_lookup_dentry+0x3c5/0x8e0 [6.1724] btrfs_lookup+0x12/0x30 [6.1731] lookup_open.isra.0+0x1aa/0x6a0 [6.1739] path_openat+0x5f7/0xc60 [6.1746] do_filp_open+0xd6/0x180 [6.1753] do_sys_openat2+0x8b/0xe0 [6.1760] __x64_sys_openat+0x54/0xa0 [6.1768] do_syscall_64+0x97/0x3e0 [6.1776] entry_SYSCALL_64_after_hwframe+0x76/0x7e [6.1784] -> #1 (btrfs-tree-00){++++}-{3:3}: [6.1794] lock_release+0x127/0x2a0 [6.1801] up_read+0x1b/0x30 [6.1808] btrfs_search_slot+0x8e0/0xff0 [6.1817] btrfs_lookup_inode+0x52/0xd0 [6.1825] __btrfs_update_delayed_inode+0x73/0x520 [6.1833] btrfs_commit_inode_delayed_inode+0x11a/0x120 [6.1842] btrfs_log_inode+0x608/0x1aa0 [6.1849] btrfs_log_inode_parent+0x249/0xf80 [6.1857] btrfs_log_dentry_safe+0x3e/0x60 [6.1865] btrfs_sync_file+0x431/0x690 [6.1872] do_fsync+0x39/0x80 [6.1879] __x64_sys_fsync+0x13/0x20 [6.1887] do_syscall_64+0x97/0x3e0 [6.1894] entry_SYSCALL_64_after_hwframe+0x76/0x7e [6.1903] -> #0 (&delayed_node->mutex){+.+.}-{3:3}: [6.1913] __lock_acquire+0x15e9/0x2820 [6.1920] lock_acquire+0xc9/0x2d0 [6.1927] __mutex_lock+0xcc/0x10a0 [6.1934] __btrfs_release_delayed_node.part.0+0x39/0x2f0 [6.1944] btrfs_evict_inode+0x20b/0x4b0 [6.1952] evict+0x15a/0x2f0 [6.1958] prune_icache_sb+0x91/0xd0 [6.1966] super_cache_scan+0x150/0x1d0 [6.1974] do_shrink_slab+0x155/0x6f0 [6.1981] shrink_slab+0x48e/0x890 [6.1988] shrink_one+0x11a/0x1f0 [6.1995] shrink_node+0xbfd/0x1320 [6.1002] balance_pgdat+0x67f/0xc60 [6.1321] kswapd+0x1dc/0x3e0 [6.1643] kthread+0xff/0x240 [6.1965] ret_from_fork+0x223/0x280 [6.1287] ret_from_fork_asm+0x1a/0x30 [6.1616] other info that might help us debug this: [6.1561] Chain exists of: &delayed_node->mutex --> btrfs-tree-00 --> fs_reclaim [6.1503] Possible unsafe locking scenario: [6.1110] CPU0 CPU1 [6.1411] ---- ---- [6.1707] lock(fs_reclaim); [6.1998] lock(btrfs-tree-00); [6.1291] lock(fs_reclaim); [6.1581] lock(&delayed_node->mutex); [6.1874] *** DEADLOCK *** [6.1716] 2 locks held by kswapd0/117: [6.1999] #0: ffffffffa4ab8ce0 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat+0x195/0xc60 [6.1294] #1: ffff8d998344b0e0 (&type->s_umount_key#40){++++}- {3:3}, at: super_cache_scan+0x37/0x1d0 [6.1596] stack backtrace: [6.1183] CPU: 11 UID: 0 PID: 117 Comm: kswapd0 Tainted: G U 6.18.0+ #4 PREEMPT(lazy) [6.1185] Tainted: [U]=USER [6.1186] Hardware name: ASUS System Product Name/PRIME B560M-A AC, BIOS 2001 02/01/2023 [6.1187] Call Trace: [6.1187] [6.1189] dump_stack_lvl+0x6e/0xa0 [6.1192] print_circular_bug.cold+0x17a/0x1c0 [6.1194] check_noncircular+0x175/0x190 [6.1197] __lock_acquire+0x15e9/0x2820 [6.1200] lock_acquire+0xc9/0x2d0 [6.1201] ? __btrfs_release_delayed_node.part.0+0x39/0x2f0 [6.1204] __mutex_lock+0xcc/0x10a0 [6.1206] ? __btrfs_release_delayed_node.part.0+0x39/0x2f0 [6.1208] ? __btrfs_release_delayed_node.part.0+0x39/0x2f0 [6.1211] ? __btrfs_release_delayed_node.part.0+0x39/0x2f0 [6.1213] __btrfs_release_delayed_node.part.0+0x39/0x2f0 [6.1215] btrfs_evict_inode+0x20b/0x4b0 [6.1217] ? lock_acquire+0xc9/0x2d0 [6.1220] evict+0x15a/0x2f0 [6.1222] prune_icache_sb+0x91/0xd0 [6.1224] super_cache_scan+0x150/0x1d0 [6.1226] do_shrink_slab+0x155/0x6f0 [6.1228] shrink_slab+0x48e/0x890 [6.1229] ? shrink_slab+0x2d2/0x890 [6.1231] shrink_one+0x11a/0x1f0 [6.1234] shrink_node+0xbfd/0x1320 [6.1236] ? shrink_node+0xa2d/0x1320 [6.1236] ? shrink_node+0xbd3/0x1320 [6.1239] ? balance_pgdat+0x67f/0xc60 [6.1239] balance_pgdat+0x67f/0xc60 [6.1241] ? finish_task_switch.isra.0+0xc4/0x2a0 [6.1246] kswapd+0x1dc/0x3e0 [6.1247] ? __pfx_autoremove_wake_function+0x10/0x10 [6.1249] ? __pfx_kswapd+0x10/0x10 [6.1250] kthread+0xff/0x240 [6.1251] ? __pfx_kthread+0x10/0x10 [6.1253] ret_from_fork+0x223/0x280 [6.1255] ? __pfx_kthread+0x10/0x10 [6.1257] ret_from_fork_asm+0x1a/0x30 [6.1260] This is because: 1) The fsync task is holding an inode's delayed node mutex (for a directory) while calling __btrfs_update_delayed_inode() and that needs to do a search on the subvolume's btree (therefore read lock some extent buffers); 2) The lookup task, at btrfs_lookup(), triggered reclaim with the GFP_KERNEL allocation done by btrfs_init_file_extent_tree() while holding a read lock on a subvolume leaf; 3) The reclaim triggered kswapd which is doing inode eviction for the directory inode the fsync task is using as an argument to btrfs_commit_inode_delayed_inode() - but in that call chain we are trying to read lock the same leaf that the lookup task is holding while calling btrfs_init_file_extent_tree() and doing the GFP_KERNEL allocation. Fix this by calling btrfs_init_file_extent_tree() after we don't need the path anymore and release it in btrfs_read_locked_inode(). Reported-by: Thomas Hellström Link: https://lore.kernel.org/linux-btrfs/6e55113a22347c3925458a5d840a18401a38b276.camel@linux.intel.com/ Fixes: 8679d2687c35 ("btrfs: initialize inode::file_extent_tree after i_mode has been set") Reviewed-by: Qu Wenruo Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index bfb8e98d8c02..5ea1c392bbc7 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -4046,11 +4046,6 @@ static int btrfs_read_locked_inode(struct btrfs_inode *inode, struct btrfs_path btrfs_set_inode_mapping_order(inode); cache_index: - ret = btrfs_init_file_extent_tree(inode); - if (ret) - goto out; - btrfs_inode_set_file_extent_range(inode, 0, - round_up(i_size_read(vfs_inode), fs_info->sectorsize)); /* * If we were modified in the current generation and evicted from memory * and then re-read we need to do a full sync since we don't have any @@ -4137,6 +4132,20 @@ cache_acl: btrfs_ino(inode), btrfs_root_id(root), ret); } + /* + * We don't need the path anymore, so release it to avoid holding a read + * lock on a leaf while calling btrfs_init_file_extent_tree(), which can + * allocate memory that triggers reclaim (GFP_KERNEL) and cause a locking + * dependency. + */ + btrfs_release_path(path); + + ret = btrfs_init_file_extent_tree(inode); + if (ret) + goto out; + btrfs_inode_set_file_extent_range(inode, 0, + round_up(i_size_read(vfs_inode), fs_info->sectorsize)); + if (!maybe_acls) cache_no_acl(vfs_inode);