On Sun, Apr 07, 2024 at 10:34:56AM +0800, Yu Kuai wrote: > Other than raw block_device fops, other filesystems can use the opened > bdev_file directly for iomap and buffer_head, and they actually don't > need to reference block_device anymore. The point here is that whether What do you mean, "reference"? The counting reference is to opened file; ->s_bdev is a cached pointer to associated struct block_device, and neither it nor pointers in buffer_head are valid past the moment when you close the file. Storing (non-counting) pointers to struct file in struct buffer_head is not different in that respect - they are *still* only valid while the "master" reference is held. Again, what's the point of storing struct file * in struct buffer_head or struct iomap? In any instances of those structures? There is a good reason to have it in places that keep a reference to opened block device - the kind that _keeps_ the device opened. Namely, there's state that need to be carried from the place where we'd opened the sucker to the place where we close it, and that state is better carried by opened file. But neither iomap nor buffer_head contain anything of that sort - the lifetime management of the opened device is not in their competence. As the matter of fact, the logics around closing those opened devices (bdev_release()) makes sure that no instances of buffer_head (or iomap) will outlive them. And they don't care about any extra state - everything they use is in block_device and coallocated inode. I could've easily missed something in one of the threads around the earlier iterations of the patchset; if that's the case, could somebody restate the rationale for that part and/or post relevant lore.kernel.org links? Christian? hch? What am I missing here?
