[PATCH] aoe 9/12: add note about the need for deadlock-free sk_buff allocation

add note about the need for deadlock-free sk_buff allocation
Signed-off-by: NEd L. Cashin <ecashin@coraid.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/aoe/todo.txt

+There is a potential for deadlock when allocating a struct sk_buff for
+data that needs to be written out to aoe storage.  If the data is
+being written from a dirty page in order to free that page, and if
+there are no other pages available, then deadlock may occur when a
+free page is needed for the sk_buff allocation.  This situation has
+not been observed, but it would be nice to eliminate any potential for
+deadlock under memory pressure.
+
+Because ATA over Ethernet is not fragmented by the kernel's IP code,
+the destructore member of the struct sk_buff is available to the aoe
+driver.  By using a mempool for allocating all but the first few
+sk_buffs, and by registering a destructor, we should be able to
+efficiently allocate sk_buffs without introducing any potential for
+deadlock.