• Andrew Morton's avatar
    [PATCH] make the pagecache lock irq-safe. · 89261aab
    Andrew Morton authored
    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.
    89261aab
swap_state.c 9.6 KB