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    Subject[BUG] slab debug vs. L1 alignement
    Currently, when enabling slab debugging, we lose the property of
    having the objects aligned on a cache line size.

    This is, imho, an error, especially if GFP_DMA is passed. Such an
    object _must_ be cache alined (and it's size rounded to a multiple
    of the cache line size).

    There is a simple performance reason on cache coherent archs, but
    there's also the fact that it will just _not_ work properly on
    non cache-coherent archs. Actually, I also have to deal with some
    old machines who have a SCSI controller who has a problem accessing
    buffers that aren't aligned on a cache line size boundary.

    This is typically causing me trouble in various parts of SCSI which
    abuses kmalloc(512, GFP_KERNEL | GFP_DMA) for buffers passed to some
    SCSI commands, typically "utility" commands used to read a disk
    capacity, read read/write protect flags, some sense buffers, etc...

    While I know SCSI shall use the consistent alloc things, it has not
    been fully fixed yet and kmalloc with GFP_DMA is still valid despite
    not beeing efficient, so we should make sure in this case, the returned
    buffer is really suitable for DMA, that is cache aligned.


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      Last update: 2005-03-22 13:47    [W:0.024 / U:5.784 seconds]
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