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SubjectRe: [BUG?] memory hotplug: include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection!
From
On Thu, Dec 16, 2010 at 10:50 PM, Gerald Schaefer
<gerald.schaefer@de.ibm.com> wrote:
> I got the same warning now after increasing /proc/sys/vm/nr_hugepages, see
> below. Both cases are easily reproducible: memory unplug with big page cache,
> or adding large pages during run-time.
>
> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 1, debug_locks = 0
> 1 lock held by bash/761:
>  #0:  (&(&inode->i_data.tree_lock)->rlock){..-.-.}, at: [<00000000002263ae>] migrate_page_move_mapping+0x4a/0x2d8
>
> stack backtrace:
> CPU: 1 Not tainted 2.6.37-rc6 #4
> Process bash (pid: 761, task: 00000000181b5540, ksp: 00000000181bb7f8)
> 00000000181bb818 00000000181bb798 0000000000000002 0000000000000000
>       00000000181bb838 00000000181bb7b0 00000000181bb7b0 000000000056bafa
>       0000000000000000 000000003f42bdf0 0000000000000002 000000001c43be30
>       000003e00000000d 000003e00000000c 00000000181bb800 0000000000000000
>       0000000000000000 0000000000100bfa 00000000181bb798 00000000181bb7d8
> Call Trace:
> ([<0000000000100b02>] show_trace+0xee/0x144)
>  [<000000000022654e>] migrate_page_move_mapping+0x1ea/0x2d8
>  [<0000000000226c80>] migrate_page+0x38/0x68
>  [<0000000000226d9a>] move_to_new_page+0xea/0x2bc
>  [<000000000022785a>] migrate_pages+0x496/0x568
>  [<000000000021e24e>] compact_zone+0x432/0x7d8
>  [<000000000021e772>] compact_zone_order+0x9e/0xbc
>  [<000000000021ed52>] try_to_compact_pages+0x1ba/0x24c
>  [<00000000001e1afa>] __alloc_pages_nodemask+0x86a/0xa64
>  [<000000000021c80c>] alloc_fresh_huge_page.clone.2+0x68/0x18c
>  [<000000000021cc4c>] set_max_huge_pages.clone.0+0xa4/0x1ac
>  [<000000000021ce06>] hugetlb_sysctl_handler+0xb2/0xcc
>  [<00000000002a6572>] proc_sys_call_handler+0xe6/0x10c
>  [<00000000002a65be>] proc_sys_write+0x26/0x34
>  [<00000000002336e0>] vfs_write+0xac/0x18c
>  [<00000000002338bc>] SyS_write+0x58/0xa8
>  [<0000000000113976>] sysc_noemu+0x16/0x1c
>  [<0000020000162edc>] 0x20000162edc
> INFO: lockdep is turned off.
>
> I honestly do not understand 100% why this is a false positive, seeing that
> e.g. find_get_page() will also use radix_tree_deref_slot(), holding only the
> rcu_read_lock, while migrate_page_move_mapping() has no rcu_read_lock() but
> the &mapping->tree_lock instead. So I'm not quite sure how to fix this
> properly, but simply adding rcu_read_lock/unlock() to the affected code paths,
> even if it is not necessary for synchronization, would get rid of the warning,
> like in the following patch. Any ideas?

In case of anon page, we hold rcu_read_lock in unmap_and_move.
The problem is file-backed page. In case of that, we hold lock_page
and mapping->tree_lock as update-side lock.
So we don't need rcu_read_lock.

>
> ---
>  fs/hugetlbfs/inode.c |    2 ++
>  mm/migrate.c         |    4 ++++
>  2 files changed, 6 insertions(+)
>
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -580,7 +580,9 @@ static int hugetlbfs_migrate_page(struct
>  {
>        int rc;
>
> +       rcu_read_lock();
>        rc = migrate_huge_page_move_mapping(mapping, newpage, page);
> +       rcu_read_unlock();
>        if (rc)
>                return rc;
>        migrate_page_copy(newpage, page);
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -417,7 +417,9 @@ int migrate_page(struct address_space *m
>
>        BUG_ON(PageWriteback(page));    /* Writeback must be complete */
>
> +       rcu_read_lock();
>        rc = migrate_page_move_mapping(mapping, newpage, page);
> +       rcu_read_unlock();
>
>        if (rc)
>                return rc;
> @@ -444,7 +446,9 @@ int buffer_migrate_page(struct address_s
>
>        head = page_buffers(page);
>
> +       rcu_read_lock();
>        rc = migrate_page_move_mapping(mapping, newpage, page);
> +       rcu_read_unlock();
>
>        if (rc)
>                return rc;
>
>
>


How about this?
Maybe Paul have better idea.
(It's apparently be word-wrapped.)

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index ab2baa5..135af1e 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -146,6 +146,20 @@ static inline void *radix_tree_deref_slot(void **pslot)
}

/**
+ * radix_tree_deref_slot_nocheck - dereference a slot without RCU check
+ * @pslot: pointer to slot, returned by radix_tree_lookup_slot
+ * Returns: item that was stored in that slot with any direct pointer flag
+ * removed.
+ *
+ * This functions works like radix_tree_deref_slot except it doesn't check
+ * RCU rule. Normally this funcion is used with update-side lock.
+ * You should use this function very carefully.
+ */
+static inline void *radix_tree_deref_slot_nocheck(void **pslot)
+{
+ return rcu_dereference_protected(*pslot, 1);
+}
+/**
* radix_tree_deref_retry - check radix_tree_deref_slot
* @arg: pointer returned by radix_tree_deref_slot
* Returns: 0 if retry is not required, otherwise retry is required
diff --git a/mm/migrate.c b/mm/migrate.c
index 2eb2243..5be2841 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -244,7 +244,8 @@ static int migrate_page_move_mapping(struct
address_space *mappin

expected_count = 2 + page_has_private(page);
if (page_count(page) != expected_count ||
- (struct page *)radix_tree_deref_slot(pslot) != page) {
+ (struct page *)radix_tree_deref_slot_nocheck(pslot)
+ != page) {
spin_unlock_irq(&mapping->tree_lock);
return -EAGAIN;
}
@@ -316,7 +317,8 @@ int migrate_huge_page_move_mapping(struct
address_space *mapping,

expected_count = 2 + page_has_private(page);
if (page_count(page) != expected_count ||
- (struct page *)radix_tree_deref_slot(pslot) != page) {
+ (struct page *)radix_tree_deref_slot_nocheck(pslot)
+ != page) {
spin_unlock_irq(&mapping->tree_lock);
return -EAGAIN;
}



--
Kind regards,
Minchan Kim
--
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