Commit 10a7a07713 ("xen: tmem: enable Xen
tmem shim to be built/loaded as a module") allows the tmem module
to be loaded any time. For this work the frontswap API had to
be able to asynchronously to call tmem_frontswap_init before
or after the swap image had been set. That was added in git
commit 905cd0e1bf
("mm: frontswap: lazy initialization to allow tmem backends to build/run as modules").
Which means we could do this (The common case):
modprobe tmem [so calls frontswap_register_ops, no ->init]
modifies tmem_frontswap_poolid = -1
swapon /dev/xvda1 [__frontswap_init, calls -> init, tmem_frontswap_poolid is
< 0 so tmem hypercall done]
Or the failing one:
swapon /dev/xvda1 [calls __frontswap_init, sets the need_init bitmap]
modprobe tmem [calls frontswap_register_ops, -->init calls, finds out
tmem_frontswap_poolid is 0, does not make a hypercall.
Later in the module_init, sets tmem_frontswap_poolid=-1]
Which meant that in the failing case we would not call the hypercall
to initialize the pool and never be able to make any frontswap
backend calls.
Moving the frontswap_register_ops after setting the tmem_frontswap_poolid
fixes it.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Bob Liu <bob.liu@oracle.com>
b2c75c446a