On Wed, 17 Oct 2007 21:59:02 -0600 [EMAIL PROTECTED] (Eric W. Biederman) wrote:
> If filesystems care at all they want absolute control over the buffer > cache. Controlling which buffers are dirty and when. Because we > keep the buffer cache in the page cache for the block device we have > not quite been giving filesystems that control leading to really weird > bugs. > > In addition this tieing of the implemetation of block device caching > and the buffer cache has resulted in a much more complicated and > limited implementation then necessary. Block devices for example > don't need buffer_heads, and it is perfectly reasonable to cache > block devices in high memory. > > To start untangling the worst of this mess this patch introduces a > second block device inode for the buffer cache. All buffer cache > operations are diverted to that use the new bd_metadata_inode, which > keeps the weirdness of the metadata requirements isolated in their > own little world. I don't think we little angels want to tread here. There are so many weirdo things out there which will break if we bust the coherence between the fs and /dev/hda1. Online resize, online fs checkers, various local tools which people have hacked up to look at metadata in a live fs, direct-io access to the underlying fs, heaven knows how many boot loader installers, etc. Cerainly I couldn't enumerate tham all. The mere thought of all this scares the crap out of me. I don't actually see what the conceptual problem is with the existing implementation. The buffer_head is a finer-grained view onto the blockdev's pagecache: it provides additional states and additional locking against a finer-grained section of the page. It works well. Yeah, the highmem thing is a bit of a problem (but waning in importance). But we can fix that by teaching individual filesystems about kmap and then tweak the blockdev's caching policy with mapping_set_gfp_mask() at mount time. If anyone cares, which they don't. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
