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    mm: add support for a filesystem to activate swap files and use direct_IO for writing swap pages · 62c230bc
    Mel Gorman 提交于
    Currently swapfiles are managed entirely by the core VM by using ->bmap to
    allocate space and write to the blocks directly.  This effectively ensures
    that the underlying blocks are allocated and avoids the need for the swap
    subsystem to locate what physical blocks store offsets within a file.
    
    If the swap subsystem is to use the filesystem information to locate the
    blocks, it is critical that information such as block groups, block
    bitmaps and the block descriptor table that map the swap file were
    resident in memory.  This patch adds address_space_operations that the VM
    can call when activating or deactivating swap backed by a file.
    
      int swap_activate(struct file *);
      int swap_deactivate(struct file *);
    
    The ->swap_activate() method is used to communicate to the file that the
    VM relies on it, and the address_space should take adequate measures such
    as reserving space in the underlying device, reserving memory for mempools
    and pinning information such as the block descriptor table in memory.  The
    ->swap_deactivate() method is called on sys_swapoff() if ->swap_activate()
    returned success.
    
    After a successful swapfile ->swap_activate, the swapfile is marked
    SWP_FILE and swapper_space.a_ops will proxy to
    sis->swap_file->f_mappings->a_ops using ->direct_io to write swapcache
    pages and ->readpage to read.
    
    It is perfectly possible that direct_IO be used to read the swap pages but
    it is an unnecessary complication.  Similarly, it is possible that
    ->writepage be used instead of direct_io to write the pages but filesystem
    developers have stated that calling writepage from the VM is undesirable
    for a variety of reasons and using direct_IO opens up the possibility of
    writing back batches of swap pages in the future.
    
    [a.p.zijlstra@chello.nl: Original patch]
    Signed-off-by: NMel Gorman <mgorman@suse.de>
    Acked-by: NRik van Riel <riel@redhat.com>
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: David S. Miller <davem@davemloft.net>
    Cc: Eric B Munson <emunson@mgebm.net>
    Cc: Eric Paris <eparis@redhat.com>
    Cc: James Morris <jmorris@namei.org>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Mike Christie <michaelc@cs.wisc.edu>
    Cc: Neil Brown <neilb@suse.de>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
    Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
    Cc: Xiaotian Feng <dfeng@redhat.com>
    Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
    62c230bc
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