[Xen-devel] [PATCH] xen/tmem: Don't over-write tmem_frontswap_poolid after tmem_frontswap_init set it.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

Commit 10a7a0771399a57a297fca9615450dbb3f88081a ("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 905cd0e1bf9ffe82d6906a01fd974ea0f70be97a
("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@xxxxxxxxxx>
---
 drivers/xen/tmem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/tmem.c b/drivers/xen/tmem.c
index cc072c6..0f0493c 100644
--- a/drivers/xen/tmem.c
+++ b/drivers/xen/tmem.c
@@ -379,10 +379,10 @@ static int xen_tmem_init(void)
 #ifdef CONFIG_FRONTSWAP
        if (tmem_enabled && frontswap) {
                char *s = "";
-               struct frontswap_ops *old_ops =
-                       frontswap_register_ops(&tmem_frontswap_ops);
+               struct frontswap_ops *old_ops;
 
                tmem_frontswap_poolid = -1;
+               old_ops = frontswap_register_ops(&tmem_frontswap_ops);
                if (IS_ERR(old_ops) || old_ops) {
                        if (IS_ERR(old_ops))
                                return PTR_ERR(old_ops);
-- 
1.8.1.4


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