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MISSING_XM_UA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; DKIM_TRACE(0.00)[suse.cz:+]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,imap1.dmz-prg2.suse.org:rdns,suse.cz:dkim,suse.cz:email,suse.com:email] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org On Wed 07-08-24 16:10:50, Stephen Zhang wrote: > Jan Kara 于2024年8月6日周二 21:40写道： > > On Sat 20-07-24 14:23:56, zhangshida wrote: > > > From: Shida Zhang > > > > > > On an old kernel version(4.19, ext3, journal=data, pagesize=64k), > > > an assertion failure will occasionally be triggered by the line below: > > > > OK, just out of curiosity, why are you using data=journal mode? It doesn't > > really get that much testing and the performance is quite bad... > > > > It is used by one of our customers. It's more like a historical issue: > About 12 years ago, they used data=journal mode for the benefit of user > data consistency brought by the mode. > Time goes by, they attempted to change, say, maybe change it to ext4 > at least, but found it is no more stable than it was under ext3... > And yeah, they decided to just leave the thing as it was and keep the system > under that state until now... I see, thanks for sharing. I was asking because we are mostly trying to steer away people from using data=journal mode and deprecate it because it adds a lot of complexity into the code without significant benefit. > > > jbd2_journal_commit_transaction > > > { > > > ... > > > J_ASSERT_BH(bh, !buffer_dirty(bh)); > > > /* > > > * The buffer on BJ_Forget list and not jbddirty means > > > ... > > > } > > > > > > AFAIC, that's how the problem works: > > > -------- > > > journal_unmap_buffer > > > jbd2_journal_invalidatepage > > > __ext4_journalled_invalidatepage > > > ext4_journalled_invalidatepage > > > do_invalidatepage > > > truncate_inode_pages_range > > > truncate_inode_pages > > > truncate_pagecache > > > ext4_setattr > > > -------- > > > > > > First try to truncate and invalidate the page. > > > Sometimes the buffer and the page won't be freed immediately. > > > the buffer will be sent to the BJ_Forget list of the currently > > > committing transaction. Maybe the transaction knows when and how > > > to free the buffer better. > > > The buffer's states now: !jbd_dirty !mapped !dirty > > > > > > Then jbd2_journal_commit_transaction(）will try to iterate over the > > > BJ_Forget list: > > > -------- > > > jbd2_journal_commit_transaction() > > > while (commit_transaction->t_forget) { > > > ... > > > } > > > -------- > > > > > > At this exact moment, another write comes: > > > -------- > > > mark_buffer_dirty > > > __block_write_begin_int > > > __block_write_begin > > > ext4_write_begin > > > -------- > > > it sees a unmapped new buffer, and marks it as dirty. > > > > This should not happen. When ext4_setattr() truncates the file, we do not > > allow reallocating these blocks for other purposes until the transaction > > ext4_setattr() will try to free it by adding it to the BJ_Forget list > for further processing. > Put it more clearly, > when ext4_setattr() truncates the file, the buffer is not fully freed > yet. It's half-freed. > Furthermore, > Because the buffer is half-freed, the reallocating thing won't need to happen. > Now, > under that scenario, can we redirty the half-freed buffer on the BJ_Forget list? > The answer may be 'yes'. > > redirty it by the following code: > ext4_block_write_begin > if (!buffer_mapped(bh)) { // check 1 > _ext4_get_block(inode, block, bh, 1); > (buffer_new(bh)) { // check 2 > if (folio_test_uptodate(folio)) { // check 3 > mark_buffer_dirty(bh); I see, right. It is not that the block would get reused. It is just that the buffer_head on the file's tail page gets reused and this causes issues. In fact, the problem is with ext4_block_write_begin() (and __block_write_begin_int()) that they call mark_buffer_dirty() on a journalled buffer before calling jbd2_journal_get_write_access() (which would remove the buffer from BJ_Forget list). This is what ultimately confuses the commit code. > For another proof, there is indeed a small window where the buffer could be > seen dirty. > Have a look at the code and comment in do_journal_get_write_access: > ---------------- > int do_journal_get_write_access(handle_t *handle, struct inode *inode, > struct buffer_head *bh) > { > ... > /* > * __block_write_begin() could have dirtied some buffers. Clean > * the dirty bit as jbd2_journal_get_write_access() could complain > * otherwise about fs integrity issues. Setting of the dirty bit > * by __block_write_begin() isn't a real problem here as we clear > * the bit before releasing a page lock and thus writeback cannot > * ever write the buffer. > */ > if (dirty) > clear_buffer_dirty(bh); // clear the dirty immdiately in case some bad > things happen OK, it was even me adding that comment 14 years ago ;) I already forgot about this nuance. So I agree with your analysis now. But still don't like adding hacks to jbd2 to acommodate for this oddity of data=journal mode. Since we already have ext4_block_write_begin() implementation anyway, we should be able to tweak it to do the right thing for data=journal mode inodes... So we could replace uses of __block_write_begin() with ext4_block_write_begin() and then call do_journal_get_write_access() in ext4_block_write_begin() for inodes with journalled data after the buffer is mapped with get_block(). >From the part: if (folio_test_uptodate(folio)) { clear_buffer_new(bh); set_buffer_uptodate(bh); mark_buffer_dirty(bh); continue; } we can actually remove the clear_buffer_new() and mark_buffer_dirty() bits because they will be done by block_commit_write() or folio_zero_new_buffers() and they are superfluous and somewhat odd here anyway. And the call to folio_zero_new_buffers() from ext4_block_write_begin() needs to call ext4_journalled_zero_new_buffers() for inodes where data is journalled. Will you try to implement this or should I look into it? Honza -- Jan Kara SUSE Labs, CR