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Date: Fri, 17 Dec 2010 08:39:12 +0000
From: Mel Gorman <mel@....ul.ie>
To: Minchan Kim <minchan.kim@...il.com>
Cc: gerald.schaefer@...ibm.com,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
paulmck@...ux.vnet.ibm.com, Milton Miller <miltonm@....com>,
linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org,
linux-ext4@...r.kernel.org, Ted Ts'o <tytso@....edu>,
Arun Bhanu <ab@...nbhanu.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [BUG?] memory hotplug: include/linux/radix-tree.h:145 invoked
rcu_dereference_check() without protection!
On Fri, Dec 17, 2010 at 09:04:13AM +0900, Minchan Kim wrote:
> On Thu, Dec 16, 2010 at 10:50 PM, Gerald Schaefer
> <gerald.schaefer@...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.)
>
heh, I wrote a patch almost identical to this and ran it overnight for testing
(test was a memory consumer running while a parallel process grew and shrunk
the hugepage pool). It passes but that is hardly a surprise. We differed
slightly in a number of respects though.
> 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);
> +}
For this, I had
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index ab2baa5..252d21c 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -146,6 +146,25 @@ static inline void *radix_tree_deref_slot(void **pslot)
}
/**
+ * radix_tree_deref_slot_protected - dereference a slot without RCU lock but with tree lock held
+ * @pslot: pointer to slot, returned by radix_tree_lookup_slot
+ * Returns: item that was stored in that slot with any direct pointer flag
+ * removed.
+ *
+ * For use with radix_tree_lookup_slot(). Caller must hold tree read
+ * locked across slot lookup and dereference. Not required if write lock is
+ * held (ie. items cannot be concurrently inserted).
+ *
+ * radix_tree_deref_retry must be used to confirm validity of the pointer if
+ * only the read lock is held.
+ */
+static inline void *radix_tree_deref_slot_protected(void **pslot,
+ spinlock_t *treelock)
+{
+ return rcu_dereference_protected(*pslot, lockdep_is_held(treelock));
+}
+
+/**
* 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
In the documentation, I noted that the check might be without RCU but with
the knowledge that it's protected by the tree lock. I'm not a RCU expert
but this is only safe when you know there isn't a parallel updater and the
treelock should be preventing that, right?
Even so, other users of rcu_dereference_protected() check a lock condition
which I used tree lock for. I intended to read through the rest of
documentation properly this morning to determine if this was indeed the
right approach.
I used the name _protected instead of _nocheck because the dereference
is still protected (by the tree lock) just not by RCU. Again, have to
check the documentation to ensure this is correct.
> +/**
> * 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;
> }
We only differed here by my passing in the &mapping->tree_lock
> @@ -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;
> }
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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