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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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