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SubjectRe: [PATCH] hugetlb: fix lockdep splat caused by pmd sharing
On Tue 30-07-13 16:58:34, Peter Zijlstra wrote:
> On Tue, Jul 30, 2013 at 04:46:00PM +0200, Michal Hocko wrote:
[...]
> > +/*
> > + * Now, reclaim path never holds hugetlbfs_inode->i_mmap_mutex while it could
> > + * hold normal inode->i_mmap_mutex so this annotation avoids a lockdep splat.
>
> How about something like:
>
> /*
> * Hugetlbfs is not reclaimable; therefore its i_mmap_mutex will never
> * be taken from reclaim -- unlike regular filesystems. This needs an
> * annotation because huge_pmd_share() does an allocation under
> * i_mmap_mutex.
> */
>
> It clarifies the exact conditions and makes easier to verify the
> validity of the annotation.

Yes, looks much better. Thanks!
---
From 673cbe2ca7df0decd7320987d97585660542e468 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.cz>
Date: Tue, 30 Jul 2013 17:22:14 +0200
Subject: [PATCH] hugetlb: fix lockdep splat caused by pmd sharing

Dave has reported the following lockdep splat:
[128095.470960] =================================
[128095.471315] [ INFO: inconsistent lock state ]
[128095.471660] 3.11.0-rc1+ #9 Not tainted
[128095.472156] ---------------------------------
[128095.472905] inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-W} usage.
[128095.473650] kswapd0/49 [HC0[0]:SC0[0]:HE1:SE1] takes:
[128095.474373] (&mapping->i_mmap_mutex){+.+.?.}, at: [<c114971b>] page_referenced+0x87/0x5e3
[128095.475128] {RECLAIM_FS-ON-W} state was registered at:
[128095.475866] [<c10a6232>] mark_held_locks+0x81/0xe7
[128095.476597] [<c10a8db3>] lockdep_trace_alloc+0x5e/0xbc
[128095.477322] [<c112316b>] __alloc_pages_nodemask+0x8b/0x9b6
[128095.478049] [<c1123ab6>] __get_free_pages+0x20/0x31
[128095.478769] [<c1123ad9>] get_zeroed_page+0x12/0x14
[128095.479477] [<c113fe1e>] __pmd_alloc+0x1c/0x6b
[128095.480138] [<c1155ea7>] huge_pmd_share+0x265/0x283
[128095.480138] [<c1155f22>] huge_pte_alloc+0x5d/0x71
[128095.480138] [<c115612e>] hugetlb_fault+0x7c/0x64a
[128095.480138] [<c114087c>] handle_mm_fault+0x255/0x299
[128095.480138] [<c15bbab0>] __do_page_fault+0x142/0x55c
[128095.480138] [<c15bbed7>] do_page_fault+0xd/0x16
[128095.480138] [<c15b927c>] error_code+0x6c/0x74
[128095.480138] irq event stamp: 3136917
[128095.480138] hardirqs last enabled at (3136917): [<c15b8139>] _raw_spin_unlock_irq+0x27/0x50
[128095.480138] hardirqs last disabled at (3136916): [<c15b7f4e>] _raw_spin_lock_irq+0x15/0x78
[128095.480138] softirqs last enabled at (3136180): [<c1048e4a>] __do_softirq+0x137/0x30f
[128095.480138] softirqs last disabled at (3136175): [<c1049195>] irq_exit+0xa8/0xaa
[128095.480138]
other info that might help us debug this:
[128095.480138] Possible unsafe locking scenario:
[128095.480138] CPU0
[128095.480138] ----
[128095.480138] lock(&mapping->i_mmap_mutex);
[128095.480138] <Interrupt>
[128095.480138] lock(&mapping->i_mmap_mutex);
[128095.480138]
*** DEADLOCK ***
[128095.480138] no locks held by kswapd0/49.
[128095.480138]
stack backtrace:
[128095.480138] CPU: 1 PID: 49 Comm: kswapd0 Not tainted 3.11.0-rc1+ #9
[128095.480138] Hardware name: Dell Inc. Precision WorkStation 490 /0DT031, BIOS A08 04/25/2008
[128095.480138] c1d32630 00000000 ee39fb18 c15b001e ee395780 ee39fb54 c15acdcb c1751845
[128095.480138] c1751bbf 00000031 00000000 00000000 00000000 00000000 00000001 00000001
[128095.480138] c1751bbf 00000008 ee395c44 00000100 ee39fb88 c10a6130 00000008 0000d8fb
[128095.480138] Call Trace:
[128095.480138] [<c15b001e>] dump_stack+0x4b/0x79
[128095.480138] [<c15acdcb>] print_usage_bug+0x1d9/0x1e3
[128095.480138] [<c10a6130>] mark_lock+0x1e0/0x261
[128095.480138] [<c10a5878>] ? check_usage_backwards+0x109/0x109
[128095.480138] [<c10a6cde>] __lock_acquire+0x623/0x17f2
[128095.480138] [<c107aa43>] ? sched_clock_cpu+0xcd/0x130
[128095.480138] [<c107a7e8>] ? sched_clock_local+0x42/0x12e
[128095.480138] [<c10a84cf>] lock_acquire+0x7d/0x195
[128095.480138] [<c114971b>] ? page_referenced+0x87/0x5e3
[128095.480138] [<c15b3671>] mutex_lock_nested+0x6c/0x3a7
[128095.480138] [<c114971b>] ? page_referenced+0x87/0x5e3
[128095.480138] [<c114971b>] ? page_referenced+0x87/0x5e3
[128095.480138] [<c11661d5>] ? mem_cgroup_charge_statistics.isra.24+0x61/0x9e
[128095.480138] [<c114971b>] page_referenced+0x87/0x5e3
[128095.480138] [<f8433030>] ? raid0_congested+0x26/0x8a [raid0]
[128095.480138] [<c112b9c7>] shrink_page_list+0x3d9/0x947
[128095.480138] [<c10a6457>] ? trace_hardirqs_on+0xb/0xd
[128095.480138] [<c112c3cf>] shrink_inactive_list+0x155/0x4cb
[128095.480138] [<c112cd07>] shrink_lruvec+0x300/0x5ce
[128095.480138] [<c112d028>] shrink_zone+0x53/0x14e
[128095.480138] [<c112e531>] kswapd+0x517/0xa75
[128095.480138] [<c112e01a>] ? mem_cgroup_shrink_node_zone+0x280/0x280
[128095.480138] [<c10661ff>] kthread+0xa8/0xaa
[128095.480138] [<c10a6457>] ? trace_hardirqs_on+0xb/0xd
[128095.480138] [<c15bf737>] ret_from_kernel_thread+0x1b/0x28
[128095.480138] [<c1066157>] ? insert_kthread_work+0x63/0x63

which is a false positive caused by hugetlb pmd sharing code which
allocates a new pmd from withing mappint->i_mmap_mutex. If this
allocation causes reclaim then the lockdep detector complains that we
might self-deadlock.

This is not correct though, because hugetlb pages are not reclaimable so
their mapping will be never touched from the reclaim path.

The patch tells lockup detector that hugetlb i_mmap_mutex is special
by assigning it a separate lockdep class so it won't report possible
deadlocks on unrelated mappings.

[peterz@infradead.org: comment for annotation]
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
---
fs/hugetlbfs/inode.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index a3f868a..3442397 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -463,6 +463,14 @@ static struct inode *hugetlbfs_get_root(struct super_block *sb,
return inode;
}

+/*
+ * Hugetlbfs is not reclaimable; therefore its i_mmap_mutex will never
+ * be taken from reclaim -- unlike regular filesystems. This needs an
+ * annotation because huge_pmd_share() does an allocation under
+ * i_mmap_mutex.
+ */
+struct lock_class_key hugetlbfs_i_mmap_mutex_key;
+
static struct inode *hugetlbfs_get_inode(struct super_block *sb,
struct inode *dir,
umode_t mode, dev_t dev)
@@ -474,6 +482,8 @@ static struct inode *hugetlbfs_get_inode(struct super_block *sb,
struct hugetlbfs_inode_info *info;
inode->i_ino = get_next_ino();
inode_init_owner(inode, dir, mode);
+ lockdep_set_class(&inode->i_mapping->i_mmap_mutex,
+ &hugetlbfs_i_mmap_mutex_key);
inode->i_mapping->a_ops = &hugetlbfs_aops;
inode->i_mapping->backing_dev_info =&hugetlbfs_backing_dev_info;
inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
--
1.8.3.2
--
Michal Hocko
SUSE Labs


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