http://bugzilla.kernel.org/show_bug.cgi?id=14354 --- Comment #177 from Chris Mason <chris.mason@xxxxxxxxxx> 2009-11-06 20:49:25 --- Created an attachment (id=23685) --> (http://bugzilla.kernel.org/attachment.cgi?id=23685) Horrible horrible debugging hack I started reading this because I was afraid the writeback changes were broken, but tracking down crc errors is really interesting as well. I started with the assumption someone was changing the buffer after we crc but before it hits disk, which in ext4 terms means that someone is probably changing data that has been sent to the log without properly diving into the jdb2 do_get_write_access() Eric helped prove this is the case by testing a patch that always crcs and writes a stable duplicate of the metadata buffer instead of the metadata buffer itself (a one liner since the log code was already setup for this). This attachment is a hack but the basic idea is to set the page for a buffer head readonly before we crc the page, and then set it read/write again when we're done with it. It will only work when the pagesize and the blocksize are the same. It probably has some false positives lurking, but so far it has found a number of places where ext4 is changing a buffer without properly calling into the jdb2 do_get_write_access() func. Eric hit a few easy ones: ext4_statfs(): /* why is statfs changing the super? */ ext4_free_blocks_count_set(es, buf->f_bfree); ext4_setxattr_set_handle() memset(raw_inode, 0, EXT4_SB(inode->i_sb)->s_inode_size); But I hit a much harder case to fix. ext4_symlink will log the data blocks that hold the symlink name (well ext4_write_begin will log them). ext4 uses the generic symlink read helpers, so when we read the link we dive into page_getlink() which uses nd_terminate_link to stuff a NULL into the end of the link's name. Since the null goes directly into the page cache page and the page was logged, we're changing things that could affect the log crcs. I think a more generic and less hacky form of this patch would be a really good thing...maybe it already exists and I just don't know about it yet ;) -- Configure bugmail: http://bugzilla.kernel.org/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are watching the assignee of the bug. -- 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
