Re: [BUG?] memory hotplug: include/linux/radix-tree.h:145 invokedrcu_dereference_check() without protection!

[Mel Gorman]

On Fri, Dec 17, 2010 at 08:39:12AM +0000, Mel Gorman wrote:
> 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@xxxxxxxxxx> 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.
> 

For completeness, this is what I tested last night. There are two "confirms"
in the changelog that I intended to work out today but maybe someone can
confirm faster.

==== CUT HERE ====
mm: migration: Use rcu_dereference_protected when dereferencing the radix tree slot during file page migration

migrate_pages() -> unmap_and_move() only calls rcu_read_lock() for anonymous
pages, as introduced by git commit 989f89c57e6361e7d16fbd9572b5da7d313b073d.
The point of the RCU protection there is part of getting a stable reference
to anon_vma and is only held for anon pages as file pages are locked
which is sufficient protection against freeing.

However, while a file page's mapping is being migrated, the radix tree
is double checked to ensure it is the expected page. This uses
radix_tree_deref_slot() -> rcu_dereference() without the RCU lock held
triggering the following warning.

[  173.674290] ===================================================
[  173.676016] [ INFO: suspicious rcu_dereference_check() usage. ]
[  173.676016] ---------------------------------------------------
[  173.676016] include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection!
[  173.676016]
[  173.676016] other info that might help us debug this:
[  173.676016]
[  173.676016]
[  173.676016] rcu_scheduler_active = 1, debug_locks = 0
[  173.676016] 1 lock held by hugeadm/2899:
[  173.676016]  #0:  (&(&inode->i_data.tree_lock)->rlock){..-.-.}, at: [<c10e3d2b>] migrate_page_move_mapping+0x40/0x1ab
[  173.676016]
[  173.676016] stack backtrace:
[  173.676016] Pid: 2899, comm: hugeadm Not tainted 2.6.37-rc5-autobuild
[  173.676016] Call Trace:
[  173.676016]  [<c128cc01>] ? printk+0x14/0x1b
[  173.676016]  [<c1063502>] lockdep_rcu_dereference+0x7d/0x86
[  173.676016]  [<c10e3db5>] migrate_page_move_mapping+0xca/0x1ab
[  173.676016]  [<c10e41ad>] migrate_page+0x23/0x39
[  173.676016]  [<c10e491b>] buffer_migrate_page+0x22/0x107
[  173.676016]  [<c10e48f9>] ? buffer_migrate_page+0x0/0x107
[  173.676016]  [<c10e425d>] move_to_new_page+0x9a/0x1ae
[  173.676016]  [<c10e47e6>] migrate_pages+0x1e7/0x2fa

This patch introduces radix_tree_deref_slot_protected() which calls
rcu_dereference_protected(). Users of it must pass in the mapping->tree_lock
that is protecting this dereference. Based on the locking hierarchy described
in mm/filemap.c, holding the tree lock is protecting the radix tree from
concurrent updaters in all cases (Confirm that no case has been missed).
According to Documentation/RCU/lockdep.txt, if there is a guarantee that
no parallel updaters exist, use of rcu_dereference_protected() is allowed
(Confirm this is accurate?).

Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
---
 include/linux/radix-tree.h |   19 +++++++++++++++++++
 mm/migrate.c               |    4 ++--
 2 files changed, 21 insertions(+), 2 deletions(-)

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
diff --git a/mm/migrate.c b/mm/migrate.c
index fe5a3c6..7d4686a 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -244,7 +244,7 @@ static int migrate_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_protected(pslot, &mapping->tree_lock) != page) {
 		spin_unlock_irq(&mapping->tree_lock);
 		return -EAGAIN;
 	}
@@ -316,7 +316,7 @@ 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_protected(pslot, &mapping->tree_lock) != page) {
 		spin_unlock_irq(&mapping->tree_lock);
 		return -EAGAIN;
 	}
--
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