On Mon 28-04-08 10:11:34, Badari Pulavarty wrote: > > On Mon, 2008-04-28 at 14:26 +0200, Jan Kara wrote: > > Hi, > > > > On Fri 25-04-08 16:38:23, Mingming Cao wrote: > > > While looking at a bug related to direct IO returns to EIO, after > > > looking at the code, I found there is a window that > > > try_to_free_buffers() from direct IO could race with JBD, which holds > > > the reference to the data buffers before journal_commit_transaction() > > > ensures the data buffers has reached to the disk. > > > > > > A little more detail: to prepare for direct IO, generic_file_direct_IO() > > > calls invalidate_inode_pages2_range() to invalidate the pages in the > > > cache before performaning direct IO. invalidate_inode_pages2_range() > > > tries to free the buffers via try_to free_buffers(), but sometimes it > > > can't, due to the buffers is possible still on some transaction's > > > t_sync_datalist or t_locked_list waiting for > > > journal_commit_transaction() to process it. > > > > > > Currently Direct IO simply returns EIO if try_to_free_buffers() finds > > > the buffer is busy, as it has no clue that JBD is referencing it. > > > > > > Is this a known issue and expected behavior? Any thoughts? > > Are you seeing this in data=ordered mode? As Andrew pointed out we do > > filemap_write_and_wait() so all the relevant data buffers of the inode > > should be already on disk. In __journal_try_to_free_buffer() we check > > whether the buffer is already-written-out data buffer and unfile and free > > it in that case. It shouldn't happen that a data buffer has > > b_next_transaction set so really the only idea why try_to_free_buffers() > > could fail is that somebody manages to write to a page via mmap before > > invalidate_inode_pages2_range() gets to it. Under which kind of load do you > > observe the problem? Do you know exactly because of which condition does > > journal_try_to_free_buffers() fail? > > > > Thank you for your reply. > > What we are noticing is invalidate_inode_pages2_range() fails with -EIO > (from try_to_free_buffers() since b_count > 0). > > I don't think the file is being updated through mmap(). Previous > writepage() added these buffers to t_sync_data list (data=ordered). > filemap_write_and_wait() waits for pagewrite back to be cleared. > So, buffers are no longer dirty, but still on the t_sync_data and > kjournald didn't get chance to process them yet :( > > Since we have elevated b_count on these buffers, try_to_free_buffers() > fails. How can we make filemap_write_and_wait() to wait for kjournald > to unfile these buffers ? Hmm, I don't get one thing: The call chain is invalidate_inode_pages2_range() -> invalidate_complete_page2() -> try_to_release_page() -> ext3_releasepage() -> journal_try_to_free_buffers() -> __journal_try_to_free_buffer() and this function should remove the buffer from the committing transaction. So who's holding the reference to those buffers? Or is it that __journal_try_to_free_buffer() fails to remove the buffer from the committing transaction? Why? Hmm, maybe I have one idea - in theory we could call __journal_try_to_free_buffer() exactly at the moment commit code inspects the buffer. Then we'd release the buffer from the transaction but try_to_free_buffers() would fail because of elevated b_count exactly as you described. Could you maybe verify this? Not that I'd know how to easily fix this ;)... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
