Re: [PATCH 2.6.11] ide-scsi: map sg buffers in idescsi_input/output_buffers() to kernel virtual address

[Jens Axboe <axboe@suse.de>]

On Wed, Mar 16 2005, Stuart_Hayes@Dell.com wrote:
> Jens Axboe wrote:
> > On Wed, Mar 16 2005, Stuart_Hayes@Dell.com wrote:
> >> Hayes, Stuart wrote:
> >>> This patch will map the sg buffers to kernel virtual memory space in
> >>> the functions idescsi_input_buffers() and idescsi_output_buffers().
> >>> Without this patch, idescsi passes a null pointer to
> >>> atapi_input_bytes() and atapi_output_bytes() when sg pages are in
> >>> high memory (i686 architecture).
> >>> 
> >>> I'm attaching as a file, too, as the text will certainly be wrapped.
> >>> 
> >>> (Sorry for the subject rename--I'm trying to use the correct format
> >>> for patch emails.) 
> >>> 
> >>> Thanks
> >>> Stuart
> >> 
> >> And, while there's another high memory/kmap patch question on this
> >> list... 
> >> 
> >> Is there some reason nobody seems interested in this patch (except
> >> Jens--thanks for the help!)?  I'm kind of new to sending in patches,
> >> and I'm not sure if I'm just not waiting long enough, or if there is
> >> a problem with this patch... 
> >> 
> >> But really, we're getting a null pointer dereference oops when using
> >> ATAPI tape drives (with ide-scsi) without this patch...
> > 
> > Sorry, that did seem to get dropped on the floor. Actually I'm
> > wondering why you are seeing highmem pages there in the first place,
> > it would be easier/better just to limit ide-scsi to non-highmem
> > pages. That would remove the need to add any work arounds in the
> > driver.    
> 
> I think we're seeing highmem pages in the sg list because that's where 
> the user memory was when st_write() was called.
> 
> The sg list is set up when st_write(), which calls setup_buffering(), 
> which calls st_map_user_pages()... this just sets up the sg pages to
> point directly to the user memory.  So, by the time ide-scsi comes into 
> the picture, the sg list is already set up to point to high memory 
> pages.
> 
> Are you suggesting that ide-scsi should change the dma_mask for the
> device so that st_map_user_pages() won't let sg pages point to high 
> memory?  Or is there something else I'm missing?

I think that is a bug, this effectively bypasses whatever restrictions
the scsi host adapter has said about memory limits. It is a problem
because the request doesn't come from the block layer (which handles all
of this).

I would much prefer fixing that real issue!

-- 
Jens Axboe