Bah, this was extremely careless of me as it's even written in teh
documentation. In this specific case, it's simply allowed to ignore whether
the RCU read lock is held or not and the BUG_ON check was unnecessary. The
tree lock protects against parallel updaters which is what we really care
about for using _protected.
In a later cycle, I should look at reducing the RCU read lock hold time
in migration. The main thing it's protecting is getting a stable
reference to anon_vma and it's held longer than is necessary for that.
In the meantime, can anyone spot a problem with this patch?
==== 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, ...
