[openib-general] FW: [PATCH 1 of 3] mad: large RMPP support

[Sean Hefty]

Roland Dreier wrote:
> My rule of thumb is that we shouldn't rely on being able to allocate a
> contiguous buffer bigger than 4 KB, but assuming we can allocate 4 KB
> is fine.  4 KB is the lowest page size of any real architecture, and
> if the kernel is out of free pages then any allocation is likely to
> fail.  Allocations of larger buffers may fail because of memory
> fragmentation, even with plenty of free memory.
> 
> That is: a 4 KB buffer is fine.

Given this, I think that we'll need to go with the linked list then.  Maybe 
something like:

struct ib_mad_segment {
	struct list_head list;
	u8 data[0];
};

struct ib_mad_send_buf {
	...
	void		*mad;	/* first segment */
	struct list_head rmpp_list;
	u32		 segment_size;
	...
};

I'm undecided about whether all MADs should use the rmpp_list, with *mad 
referencing the data of the first segment.  This keeps the code consistent, but 
would result in the first segment being larger (256-bytes) than additional 
segments (say 220-bytes).

Users could then walk the list of buffers without calling a routine that needs 
to start at the beginning of the list every time.

- Sean