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@linaro.org Closes: https://lore.kernel.org/lkml/63903521.5040307@huawei.com/t/ Reported-by: syzbot+a158d886ca08a3fecca4@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?id=899b37f20ce4072bcdfecfe1647b39602e956e3... Fixes: 7b4cc9787fe3 ("ext4: evict inline data when writing to memory map") CC: stable@vger.kernel.org # 4.12+ Signed-off-by: Baokun Li libaokun1@huawei.com
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. And when you do write(2), you hold inode_lock() and then you copy data from the use provided buffer to the pagecache pages and that can cause a page fault on the user provided buffer which will try to grab task->mm->mmap_lock.
This lock inversion is the main reason why inode lock cannot be used anywhere in the page fault path.
Honza