• Dave Chinner's avatar
    xfs: rework remote attr CRCs · ad1858d7
    Dave Chinner authored
    Note: this changes the on-disk remote attribute format. I assert
    that this is OK to do as CRCs are marked experimental and the first
    kernel it is included in has not yet reached release yet. Further,
    the userspace utilities are still evolving and so anyone using this
    stuff right now is a developer or tester using volatile filesystems
    for testing this feature. Hence changing the format right now to
    save longer term pain is the right thing to do.
    
    The fundamental change is to move from a header per extent in the
    attribute to a header per filesytem block in the attribute. This
    means there are more header blocks and the parsing of the attribute
    data is slightly more complex, but it has the advantage that we
    always know the size of the attribute on disk based on the length of
    the data it contains.
    
    This is where the header-per-extent method has problems. We don't
    know the size of the attribute on disk without first knowing how
    many extents are used to hold it. And we can't tell from a
    mapping lookup, either, because remote attributes can be allocated
    contiguously with other attribute blocks and so there is no obvious
    way of determining the actual size of the atribute on disk short of
    walking and mapping buffers.
    
    The problem with this approach is that if we map a buffer
    incorrectly (e.g. we make the last buffer for the attribute data too
    long), we then get buffer cache lookup failure when we map it
    correctly. i.e. we get a size mismatch on lookup. This is not
    necessarily fatal, but it's a cache coherency problem that can lead
    to returning the wrong data to userspace or writing the wrong data
    to disk. And debug kernels will assert fail if this occurs.
    
    I found lots of niggly little problems trying to fix this issue on a
    4k block size filesystem, finally getting it to pass with lots of
    fixes. The thing is, 1024 byte filesystems still failed, and it was
    getting really complex handling all the corner cases that were
    showing up. And there were clearly more that I hadn't found yet.
    
    It is complex, fragile code, and if we don't fix it now, it will be
    complex, fragile code forever more.
    
    Hence the simple fix is to add a header to each filesystem block.
    This gives us the same relationship between the attribute data
    length and the number of blocks on disk as we have without CRCs -
    it's a linear mapping and doesn't require us to guess anything. It
    is simple to implement, too - the remote block count calculated at
    lookup time can be used by the remote attribute set/get/remove code
    without modification for both CRC and non-CRC filesystems. The world
    becomes sane again.
    
    Because the copy-in and copy-out now need to iterate over each
    filesystem block, I moved them into helper functions so we separate
    the block mapping and buffer manupulations from the attribute data
    and CRC header manipulations. The code becomes much clearer as a
    result, and it is a lot easier to understand and debug. It also
    appears to be much more robust - once it worked on 4k block size
    filesystems, it has worked without failure on 1k block size
    filesystems, too.
    Signed-off-by: default avatarDave Chinner <dchinner@redhat.com>
    Reviewed-by: default avatarBen Myers <bpm@sgi.com>
    Signed-off-by: default avatarBen Myers <bpm@sgi.com>
    ad1858d7
xfs_attr_remote.h 1.93 KB