On Wed, Dec 18, 2013 at 09:07:59PM -0500, Theodore Ts'o wrote: > On Wed, Dec 18, 2013 at 07:27:49AM -0700, Matthew Wilcox wrote: > > > > I think there is a callback in xip_file_write(), and it's get_xip_mem(). > > From what you're saying, it sounds like it's just not doing enough. > > The problem is that git_xip_mem() is called before we write to the > memory, right? > > We need to convert the uninit extents to be marked as initialized in > *after* the write has been sent to the storage medium. Now that I've spent the best part of a day looking at the ext4 code, I still don't think there's a problem here. With the way the XIP code is currently written (calling ext4_get_block with create=1), we won't get an uninitialised extent in the caller. Instead, we'll get one that's been zeroed (the zeroing is part of patch 3/3 and done only for xip files). As I understand it, when ext4 uses direct I/O, it can pass ext4_get_block_write() as the get_block method, which uses the magic EXT4_GET_BLOCKS_IO_CREATE_EXT flag to permit the allocation of uninitialised extents. But the regular ext4_get_block cannot create uninitialised extents (it can return them in the case of create=0, and we handle that correctly as a hole). Moreover, I don't see, eg, block_read_full_page() handling uninitialised extents specially, so I'm pretty sure the regular ext4_get_block() can't create uninitialised extents. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- 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
