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    SubjectRe: cdrom: use kmalloced buffers instead of buffers on stack
    On Tue, Apr 22 2008, FUJITA Tomonori wrote:
    > On Tue, 22 Apr 2008 07:48:58 +0200
    > tsbogend@alpha.franken.de (Thomas Bogendoerfer) wrote:
    >
    > > On Mon, Apr 21, 2008 at 10:01:26PM -0400, Jeff Garzik wrote:
    > > > > cdrom: use kmalloced buffers instead of buffers on stack
    > > > >
    > > > > If cdrom commands are issued to a scsi drive in most cases the buffer
    > > > > will be
    > > > > filled via dma. This leads to bad stack corruption on non coherent
    > > > > platforms,
    > > > > because the buffers are neither cache line aligned nor is the size a
    > > > > multiple
    > > > > of the cache line size. Using kmalloced buffers avoids this.
    > > > >
    > > > > Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
    > > > > Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
    > > > >---
    > > > > drivers/cdrom/cdrom.c | 274
    > > > > ++++++++++++++++++++++++++++++++-----------------
    > > > > 1 files changed, 181 insertions(+), 93 deletions(-)
    > > >
    > > > Eh... AFAICS this is only really useful in two of the cases converted.
    > > >
    > > > For all the other cases (<= 32 bytes), it is _far_ less complex, far
    > > > less code to simply communicate the additional alignment requirements to
    > > > the compiler.
    > > >
    > > > What about __attribute__ __aligned__? Was that tried?
    > >
    > > I used that while narrowing down the bug. But not only the alignment is
    > > important, but also size needs to be a multiple of the cache line size.
    > > Which means it needs to be 128 bytes for most SGI machines. That
    > > and the following in DMA-mapping.txt
    > >
    > > "This rule also means that you may use neither kernel image addresses
    > > (items in data/text/bss segments), nor module image addresses, nor
    > > stack addresses for DMA."
    > >
    > > let me choose the kmalloc() solution.
    >
    > Can we advertise such architecture's dma restrictions? For example, if
    > we can update dma_pad_mask and dma_alignment in request_queue,
    > blk_rq_map_kern uses a proper bounce buffer for such
    > architectures. Then we can avoid putting extra complexity in uppper
    > drivers such as cdrom.c

    That would work fine, if cdrom was then also updated to get rid of
    ->generic_packet() and use the regular queue transport instead. Which
    would be a VERY nice thing to do anyway, so I'd welcome the effort :-)

    --
    Jens Axboe



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