On Tue 30-05-23 17:39:16, Baokun Li wrote: > On 2023/5/30 15:42, Jan Kara wrote: > > On Tue 30-05-23 10:00:44, Baokun Li wrote: > > > On 2023/5/29 22:44, Jan Kara wrote: > > > > On Mon 29-05-23 16:01:48, Baokun Li wrote: > > > > > Syzbot reported a BUG_ON: > > > > > ================================================================== > > > > > EXT4-fs (loop0): mounted filesystem without journal. Quota mode: none. > > > > > EXT4-fs error (device loop0): ext4_mb_generate_buddy:1098: group 0, block > > > > > bitmap and bg descriptor inconsistent: 25 vs 150994969 free clusters > > > > > ------------[ cut here ]------------ > > > > > kernel BUG at fs/ext4/ext4_jbd2.c:53! > > > > > invalid opcode: 0000 [#1] PREEMPT SMP KASAN > > > > > CPU: 1 PID: 494 Comm: syz-executor.0 6.1.0-rc7-syzkaller-ga4412fdd49dc #0 > > > > > RIP: 0010:__ext4_journal_stop+0x1b3/0x1c0 > > > > > [...] > > > > > Call Trace: > > > > > ext4_write_inline_data_end+0xa39/0xdf0 > > > > > ext4_da_write_end+0x1e2/0x950 > > > > > generic_perform_write+0x401/0x5f0 > > > > > ext4_buffered_write_iter+0x35f/0x640 > > > > > ext4_file_write_iter+0x198/0x1cd0 > > > > > vfs_write+0x8b5/0xef0 > > > > > [...] > > > > > ================================================================== > > > > > > > > > > The above BUG_ON is triggered by the following race: > > > > > > > > > > cpu1 cpu2 > > > > > ________________________|________________________ > > > > > ksys_write > > > > > vfs_write > > > > > new_sync_write > > > > > ext4_file_write_iter > > > > > ext4_buffered_write_iter > > > > > generic_perform_write > > > > > ext4_da_write_begin > > > > > do_fault > > > > > do_page_mkwrite > > > > > ext4_page_mkwrite > > > > > ext4_convert_inline_data > > > > > ext4_convert_inline_data_nolock > > > > > ext4_destroy_inline_data_nolock > > > > > //clear EXT4_STATE_MAY_INLINE_DATA > > > > > ext4_map_blocks --> return error > > > > > ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA) > > > > > ext4_block_write_begin > > > > > ext4_restore_inline_data > > > > > // set EXT4_STATE_MAY_INLINE_DATA > > > > > ext4_da_write_end > > > > > ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA) > > > > > ext4_write_inline_data_end > > > > > handle=NULL > > > > > ext4_journal_stop(handle) > > > > > __ext4_journal_stop > > > > > ext4_put_nojournal(handle) > > > > > ref_cnt = (unsigned long)handle > > > > > BUG_ON(ref_cnt == 0) ---> BUG_ON > > > > > > > > > > The root cause of this problem is that the ext4_convert_inline_data() in > > > > > ext4_page_mkwrite() does not grab i_rwsem, so it may race with > > > > > ext4_buffered_write_iter() and cause the write_begin() and write_end() > > > > > functions to be inconsistent and trigger BUG_ON. > > > > > > > > > > To solve the above issue, we cannot add inode_lock directly to > > > > > ext4_page_mkwrite(), because this function is a hot path and frequent calls > > > > > to inode_lock will cause performance degradation for multi-threaded reads > > > > > and writes. Hence, we move ext4_convert_inline_data() to ext4_file_mmap(), > > > > > and only when inline_data is enabled and mmap a file in shared write mode, > > > > > we hold the lock to convert, which can reduce the impact on performance. > > > > > > > > > > Reported-by: Jun Nie <jun.nie@xxxxxxxxxx> > > > > > Closes: https://lore.kernel.org/lkml/63903521.5040307@xxxxxxxxxx/t/ > > > > > Reported-by: syzbot+a158d886ca08a3fecca4@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > Closes: https://syzkaller.appspot.com/bug?id=899b37f20ce4072bcdfecfe1647b39602e956e36 > > > > > Fixes: 7b4cc9787fe3 ("ext4: evict inline data when writing to memory map") > > > > > CC: stable@xxxxxxxxxxxxxxx # 4.12+ > > > > > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx> > > > > Thanks for the patch! The problem with i_rwsem in ext4_page_mkwrite() is > > > > not so much about performance as about lock ordering. In > > > > ext4_page_mkwrite() we are called with mmap_sem held and so we cannot > > > > acquire i_rwsem because it ranks about it. > > > Thank you for your review! > > > > > > I'm sorry I didn't make myself clear here. > > > > > > Yes, we can't get i_rwsem after holding mmap_sem at any time, otherwise > > > ABBA deadlock may occur. The "add inode_lock directly" in my patch > > > description actually looks like this: > > > ``` > > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > > > index b98d2d58b900..c9318dc2a613 100644 > > > --- a/fs/ext4/inode.c > > > +++ b/fs/ext4/inode.c > > > @@ -6025,12 +6025,14 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf) > > > sb_start_pagefault(inode->i_sb); > > > file_update_time(vma->vm_file); > > > > > > - filemap_invalidate_lock_shared(mapping); > > > - > > > + inode_lock(inode); > > > err = ext4_convert_inline_data(inode); > > > + inode_unlock(inode); > > > if (err) > > > goto out_ret; > > > > > > + filemap_invalidate_lock_shared(mapping); > > > + > > > /* > > > * On data journalling we skip straight to the transaction handle: > > > * there's no delalloc; page truncated will be checked later; the > > > ``` > > Yes, but even this could deadlock. The ABBA deadlock I'm speaking about > > would not be created with mapping->invalidate_lock but rather with > > task->mm->mmap_lock which is acquired at the beginning of page fault in the > > arch code. > > Thanks for the explanation! > > I thought the mmap_sem said was "&EXT4_I(inode)->i_mmap_sem". We don't have that one anymore :). But yes, there are too many mmap locks so I should have been more precise from the start. I'm sorry for the confusion. > But here are some more questions: > > 1) In the arch code page fault is handled by mmap_read_lock(mm) to get the > shared lock, why would this lead to ABBA deadlock? > > 2) Why would page fault be triggered in the write process? > > Could you explain it in more detail? OK, let me draw here the exact deadlock graph: CPU0 CPU1 CPU2 write(2) page fault mmap(2) do_user_addr_fault() mmap_read_lock() ... ... vm_mmap_pgoff() mmap_write_lock_killable(mm) - blocks on CPU1 holding the lock ext4_buffered_write_iter() inode_lock(inode); ... generic_perform_write(iocb, from); fault_in_iov_iter_readable(i, bytes) - hits pagefault do_user_addr_fault() mmap_read_lock() - blocks on CPU2 as writer queued first ext4_page_mkwrite() - would deadlock here if we try to take inode lock Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
