On Tue, Sep 08, 2009 at 03:26:01PM +0200, Jan Kara wrote: > Hi, > > When looking at how ext3/4 handles fsync, I've realized I don't > understand how writing out inode on fsync can work. The problem is that > ext3/4 mostly calls ext?_mark_inode_dirty() which actually does *not* dirty > the inode. It just copies the in-memory inode content to disk buffer. > So in particular the inode looks clean to VFS and our check in > ext?_sync_file() shouldn't trigger. > The only obvious case when we call mark_inode_dirty() is from write_end > functions when we update i_size but that's clearly not enough. Now I did > some research why things seem to be actually working. The trick is that > when allocating block, we call vfs_dq_alloc_block() which calls > mark_inode_dirty(). But that's all what's keeping our fsync / writeout > logic from breaking! ext4_handle_dirty_metadata should do mark_inode_dirty right ? __ext4_handle_dirty_metadata -> mark_buffer_dirty ->__set_page_dirty -> __mark_inode_dirty -> list_move(&inode->i_list, &sb->s_dirty); > There are even some cases when the logic actually is broken (I've tested > it and it really does not work) - for example when you create an empty > file, the inode won't get written when you fsync it. > So what we should IMHO do is to convert all ext?_mark_inode_dirty() > calls to simple mark_inode_dirty() (or even maybe introduce and use > mark_inode_dirty_datasync() where appropriate). It will cost us some more > CPU and stack space but if we optimize ext3_dirty_inode() for the case > where handle is already started, it shouldn't be too bad. -aneesh -- 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
