[PATCH] make the pagecache lock irq-safe.
Intro to these patches: - Major surgery against the pagecache, radix-tree and writeback code. This work is to address the O_DIRECT-vs-buffered data exposure horrors which we've been struggling with for months. As a side-effect, 32 bytes are saved from struct inode and eight bytes are removed from struct page. At a cost of approximately 2.5 bits per page in the radix tree nodes on 4k pagesize, assuming the pagecache is densely populated. Not all pages are pagecache; other pages gain the full 8 byte saving. This change will break any arch code which is using page->list and will also break any arch code which is using page->lru of memory which was obtained from slab. The basic problem which we (mainly Daniel McNeil) have been struggling with is in getting a really reliable fsync() across the page lists while other processes are performing writeback against the same file. It's like juggling four bars of wet soap with your eyes shut while someone is whacking you with a baseball bat. Daniel pretty much has the problem plugged but I suspect that's just because we don't have testcases to trigger the remaining problems. The complexity and additional locking which those patches add is worrisome. So the approach taken here is to remove the page lists altogether and replace the list-based writeback and wait operations with in-order radix-tree walks. The radix-tree code has been enhanced to support "tagging" of pages, for later searches for pages which have a particular tag set. This means that we can ask the radix tree code "find me the next 16 dirty pages starting at pagecache index N" and it will do that in O(log64(N)) time. This affects I/O scheduling potentially quite significantly. It is no longer the case that the kernel will submit pages for I/O in the order in which the application dirtied them. We instead submit them in file-offset order all the time. This is likely to be advantageous when applications are seeking all over a large file randomly writing small amounts of data. I haven't performed much benchmarking, but tiobench random write throughput seems to be increased by 30%. Other tests appear to be unaltered. dbench may have got 10-20% quicker, but it's variable. There is one large file which everyone seeks all over randomly writing small amounts of data: the blockdev mapping which caches filesystem metadata. The kernel's IO submission patterns for this are now ideal. Because writeback and wait-for-writeback use a tree walk instead of a list walk they are no longer livelockable. This probably means that we no longer need to hold i_sem across O_SYNC writes and perhaps fsync() and fdatasync(). This may be beneficial for databases: multiple processes writing and syncing different parts of the same file at the same time can now all submit and wait upon writes to just their own little bit of the file, so we can get a lot more data into the queues. It is trivial to implement a part-file-fdatasync() as well, so applications can say "sync the file from byte N to byte M", and multiple applications can do this concurrently. This is easy for ext2 filesystems, but probably needs lots of work for data-journalled filesystems and XFS and it probably doesn't offer much benefit over an i_semless O_SYNC write. These patches can end up making ext3 (even) slower: for i in 1 2 3 4 do dd if=/dev/zero of=$i bs=1M count=2000 & done runs awfully slow on SMP. This is, yet again, because all the file blocks are jumbled up and the per-file linear writeout causes tons of seeking. The above test runs sweetly on UP because the on UP we don't allocate blocks to different files in parallel. Mingming and Badari are working on getting block reservation working for ext3 (preallocation on steroids). That should fix ext3 up. This patch: - Later, we'll need to access the radix trees from inside disk I/O completion handlers. So make mapping->page_lock irq-safe. And rename it to tree_lock to reliably break any missed conversions.
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