On 2020/1/8 下午8:28, Nikolay Borisov wrote: > > > On 8.01.20 г. 7:12 ч., Qu Wenruo wrote: >> [BUG] >> There are several different KASAN reports for balance + snapshot >> workloads. >> Involved call paths include: >> >> should_ignore_root+0x54/0xb0 [btrfs] >> build_backref_tree+0x11af/0x2280 [btrfs] >> relocate_tree_blocks+0x391/0xb80 [btrfs] >> relocate_block_group+0x3e5/0xa00 [btrfs] >> btrfs_relocate_block_group+0x240/0x4d0 [btrfs] >> btrfs_relocate_chunk+0x53/0xf0 [btrfs] >> btrfs_balance+0xc91/0x1840 [btrfs] >> btrfs_ioctl_balance+0x416/0x4e0 [btrfs] >> btrfs_ioctl+0x8af/0x3e60 [btrfs] >> do_vfs_ioctl+0x831/0xb10 >> ksys_ioctl+0x67/0x90 >> __x64_sys_ioctl+0x43/0x50 >> do_syscall_64+0x79/0xe0 >> entry_SYSCALL_64_after_hwframe+0x49/0xbe >> >> create_reloc_root+0x9f/0x460 [btrfs] >> btrfs_reloc_post_snapshot+0xff/0x6c0 [btrfs] >> create_pending_snapshot+0xa9b/0x15f0 [btrfs] >> create_pending_snapshots+0x111/0x140 [btrfs] >> btrfs_commit_transaction+0x7a6/0x1360 [btrfs] >> btrfs_mksubvol+0x915/0x960 [btrfs] >> btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs] >> btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs] >> btrfs_ioctl+0x241b/0x3e60 [btrfs] >> do_vfs_ioctl+0x831/0xb10 >> ksys_ioctl+0x67/0x90 >> __x64_sys_ioctl+0x43/0x50 >> do_syscall_64+0x79/0xe0 >> entry_SYSCALL_64_after_hwframe+0x49/0xbe >> >> btrfs_reloc_pre_snapshot+0x85/0xc0 [btrfs] >> create_pending_snapshot+0x209/0x15f0 [btrfs] >> create_pending_snapshots+0x111/0x140 [btrfs] >> btrfs_commit_transaction+0x7a6/0x1360 [btrfs] >> btrfs_mksubvol+0x915/0x960 [btrfs] >> btrfs_ioctl_snap_create_transid+0x1d5/0x1e0 [btrfs] >> btrfs_ioctl_snap_create_v2+0x1d3/0x270 [btrfs] >> btrfs_ioctl+0x241b/0x3e60 [btrfs] >> do_vfs_ioctl+0x831/0xb10 >> ksys_ioctl+0x67/0x90 >> __x64_sys_ioctl+0x43/0x50 >> do_syscall_64+0x79/0xe0 >> entry_SYSCALL_64_after_hwframe+0x49/0xbe >> >> [CAUSE] >> All these call sites are only relying on root->reloc_root, which can >> undergo btrfs_drop_snapshot(), and since we don't have real refcount > > what do you mean by "root->reloc_root can undergo btrfs_drop_snapshot" ? I mean some caller got root->reloc_root and use it, while root->reloc_root soon get dropped by btrfs_drop_snapshot(). > >> based protection to reloc roots, we can reach already dropped reloc >> root, triggering KASAN. > what's the relationship between not having a refcount protection and > reaching reloc roots, perhaps you could expand the explanation? If we had a proper refcount protection, we could wait until we're the last holder of reloc_root before calling btrfs_drop_snapshot(). And to me, that should be the correct solution, while this patch is just a quick and maybe dirty fix mostly for backport. > >> >> [FIX] >> To avoid such access to unstable root->reloc_root, we should check >> BTRFS_ROOT_DEAD_RELOC_TREE bit first. >> >> This patch introduces a new wrapper, have_reloc_root(), to do the proper >> check for most callers who don't distinguish merged reloc tree and no >> reloc tree. >> >> The only exception is should_ignore_root(), as merged reloc tree can be >> ignored, while no reloc tree shouldn't. >> >> [CRITICAL SECTION ANALYSE] >> Although test_bit()/set_bit()/clear_bit() doesn't imply a barrier, the >> DEAD_RELOC_TREE bit has extra help from transaction as a higher level >> barrier, the lifespan of root::reloc_root and DEAD_RELOC_TREE bit are: >> >> NULL: reloc_root is NULL PTR: reloc_root is not NULL >> 0: DEAD_RELOC_ROOT bit not set DEAD: DEAD_RELOC_ROOT bit set >> >> (NULL, 0) Initial state __ >> | /\ Section A >> btrfs_init_reloc_root() \/ >> | __ >> (PTR, 0) reloc_root initialized /\ >> | | >> btrfs_update_reloc_root() | Section B >> | | >> (PTR, DEAD) reloc_root has been merged \/ >> | __ >> === btrfs_commit_transaction() ==================== >> | /\ >> clean_dirty_subvols() | >> | | Section C >> (NULL, DEAD) reloc_root cleanup starts \/ >> | __ >> btrfs_drop_snapshot() /\ >> | | Section D >> (NULL, 0) Back to initial state \/ >> >> Very have_reloc_root() or test_bit(DEAD_RELOC_ROOT) caller has hold a > > ^^ Perhaps you meant: Every caller of have_reloc_root or > test_bit(DED_RELOC_ROOT) holds a transaction handle which ensures > modifications in those function are limited to a single transaction? Yep, I mean *E*very. It looks I need to replace the switch of my 'e' key... Thanks, Qu
