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    This commit enables a UFS filesystem to do a forcible unmount when · c18fd016
    chs authored
    the underlying media fails or becomes inaccessible. For example
    when a USB flash memory card hosting a UFS filesystem is unplugged.
    The strategy for handling disk I/O errors when soft updates are
    enabled is to stop writing to the disk of the affected file system
    but continue to accept I/O requests and report that all future
    writes by the file system to that disk actually succeed. Then
    initiate an asynchronous forced unmount of the affected file system.
    There are two cases for disk I/O errors:
       - ENXIO, which means that this disk is gone and the lower layers
         of the storage stack already guarantee that no future I/O to
         this disk will succeed.
       - EIO (or most other errors), which means that this particular
         I/O request has failed but subsequent I/O requests to this
         disk might still succeed.
    For ENXIO, we can just clear the error and continue, because we
    know that the file system cannot affect the on-disk state after we
    see this error. For EIO or other errors, we arrange for the geom_vfs
    layer to reject all future I/O requests with ENXIO just like is
    done when the geom_vfs is orphaned. In both cases, the file system
    code can just clear the error and proceed with the forcible unmount.
    This new treatment of I/O errors is needed for writes of any buffer
    that is involved in a dependency. Most dependencies are described
    by a structure attached to the buffer's b_dep field. But some are
    created and processed as a result of the completion of the dependencies
    attached to the buffer.
    Clearing of some dependencies require a read. For example if there
    is a dependency that requires an inode to be written, the disk block
    containing that inode must be read, the updated inode copied into
    place in that buffer, and the buffer then written back to disk.
    Often the needed buffer is already in memory and can be used. But
    if it needs to be read from the disk, the read will fail, so we
    fabricate a buffer full of zeroes and pretend that the read succeeded.
    This zero'ed buffer can be updated and written back to disk.
    The only case where a buffer full of zeros causes the code to do
    the wrong thing is when reading an inode buffer containing an inode
    that still has an inode dependency in memory that will reinitialize
    the effective link count (i_effnlink) based on the actual link count
    (i_nlink) that we read. To handle this case we now store the i_nlink
    value that we wrote in the inode dependency so that it can be
    restored into the zero'ed buffer thus keeping the tracking of the
    inode link count consistent.
    Because applications depend on knowing when an attempt to write
    their data to stable storage has failed, the fsync(2) and msync(2)
    system calls need to return errors if data fails to be written to
    stable storage. So these operations return ENXIO for every call
    made on files in a file system where we have otherwise been ignoring
    I/O errors.
    Coauthered by: mckusick
    Reviewed by:   kib
    Tested by:     Peter Holm
    Approved by:   mckusick (mentor)
    Sponsored by:  Netflix
    Differential Revision: