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SubjectRe: [Update][PATCH v5 7/9] mm/swap: Add cache for swap slots allocation
On Tue, Jan 17, 2017 at 10:55:47AM +0800, Huang, Ying wrote:
> Hi, Andrew,
>
> This update patch is to fix the preemption warning raised by Michal
> Hocko. raw_cpu_ptr() is used to replace this_cpu_ptr() and comments are
> added for why it is used.
>

Andrew & Michal,

Here's a fix that's a follow on patch instead of an updated patch
as Michal has suggested. I've updated the comments a bit to make it
clearer.

Thanks.

Tim

--->8---
Date: Tue, 17 Jan 2017 12:57:00 -0800
Subject: [PATCH] mm/swap: Use raw_cpu_ptr over this_cpu_ptr for swap slots
access
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Ying Huang <ying.huang@intel.com>, dave.hansen@intel.com, ak@linux.intel.com, aaron.lu@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins <hughd@google.com>, Shaohua Li <shli@kernel.org>, Minchan Kim <minchan@kernel.org>, Rik van Riel <riel@redhat.com>, Andrea Arcangeli <aarcange@redhat.com>, Kirill A . Shutemov <kirill.shutemov@linux.intel.com>, Vladimir Davydov <vdavydov.dev@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>, Hillf Danton <hillf.zj@alibaba-inc.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Jonathan Corbet <corbet@lwn.net>

From: "Huang, Ying" <ying.huang@intel.com>

The usage of this_cpu_ptr in get_swap_page causes a bug warning
as it is used in pre-emptible code.

[ 57.812314] BUG: using smp_processor_id() in preemptible [00000000] code: kswapd0/527
[ 57.814360] caller is debug_smp_processor_id+0x17/0x19
[ 57.815237] CPU: 1 PID: 527 Comm: kswapd0 Tainted: G W 4.9.0-mmotm-00135-g4e9a9895ebef #1042
[ 57.816019] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.1-1 04/01/2014
[ 57.816019] ffffc900001939c0 ffffffff81329c60 0000000000000001 ffffffff81a0ce06
[ 57.816019] ffffc900001939f0 ffffffff81343c2a 00000000000137a0 ffffea0000dfd2a0
[ 57.816019] ffff88003c49a700 ffffc90000193b10 ffffc90000193a00 ffffffff81343c53
[ 57.816019] Call Trace:
[ 57.816019] [<ffffffff81329c60>] dump_stack+0x68/0x92
[ 57.816019] [<ffffffff81343c2a>] check_preemption_disabled+0xce/0xe0
[ 57.816019] [<ffffffff81343c53>] debug_smp_processor_id+0x17/0x19
[ 57.816019] [<ffffffff8115f06f>] get_swap_page+0x19/0x183
[ 57.816019] [<ffffffff8114e01d>] shmem_writepage+0xce/0x38c
[ 57.816019] [<ffffffff81148916>] shrink_page_list+0x81f/0xdbf
[ 57.816019] [<ffffffff81149652>] shrink_inactive_list+0x2ab/0x594
[ 57.816019] [<ffffffff8114a22f>] shrink_node_memcg+0x4c7/0x673
[ 57.816019] [<ffffffff8114a49f>] shrink_node+0xc4/0x282
[ 57.816019] [<ffffffff8114a49f>] ? shrink_node+0xc4/0x282
[ 57.816019] [<ffffffff8114b8cb>] kswapd+0x656/0x834

Logic wise, We do allow pre-emption as per cpu ptr cache->slots is
protected by the mutex cache->alloc_lock. We switch the
inappropriately used this_cpu_ptr to raw_cpu_ptr for per cpu ptr
access of cache->slots.

Reported-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
mm/swap_slots.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/swap_slots.c b/mm/swap_slots.c
index 8cf941e..9b5bc86 100644
--- a/mm/swap_slots.c
+++ b/mm/swap_slots.c
@@ -303,7 +303,16 @@ swp_entry_t get_swap_page(void)
swp_entry_t entry, *pentry;
struct swap_slots_cache *cache;

- cache = this_cpu_ptr(&swp_slots);
+ /*
+ * Preemption is allowed here, because we may sleep
+ * in refill_swap_slots_cache(). But it is safe, because
+ * accesses to the per-CPU data structure are protected by the
+ * mutex cache->alloc_lock.
+ *
+ * The alloc path here does not touch cache->slots_ret
+ * so cache->free_lock is not taken.
+ */
+ cache = raw_cpu_ptr(&swp_slots);

entry.val = 0;
if (check_cache_active()) {
--
2.5.5
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