On Wed, Jul 17, 2019 at 02:02:20PM +0200, Jan Stancek wrote:
> LTP mtest06 has been observed to rarely hit "still mapped when deleted"
> and following BUG_ON on arm64:
> page:ffff7e02fa37e480 refcount:3 mapcount:1 mapping:ffff80be3d678ab0
> index:0x0
> xfs_address_space_operations [xfs]
> flags: 0xbfffe000000037(locked|referenced|uptodate|lru|active)
> page dumped because: VM_BUG_ON_PAGE(page_mapped(page))
> ------------[ cut here ]------------
> kernel BUG at mm/filemap.c:171!
> Internal error: Oops - BUG: 0 [#1] SMP
> CPU: 220 PID: 154292 Comm: mmap1 Not tainted 5.2.0-0ecfebd.cki #1
> Hardware name: HPE Apollo 70 /C01_APACHE_MB , BIOS L50_5.13_1.10 05/17/2019
> pstate: 40400089 (nZcv daIf +PAN -UAO)
> pc : unaccount_page_cache_page+0x17c/0x1a0
> lr : unaccount_page_cache_page+0x17c/0x1a0
> Call trace:
> unaccount_page_cache_page+0x17c/0x1a0
> delete_from_page_cache_batch+0xa0/0x300
> truncate_inode_pages_range+0x1b8/0x640
> truncate_inode_pages_final+0x88/0xa8
> evict+0x1a0/0x1d8
> iput+0x150/0x240
> dentry_unlink_inode+0x120/0x130
> __dentry_kill+0xd8/0x1d0
> dentry_kill+0x88/0x248
> dput+0x168/0x1b8
> __fput+0xe8/0x208
> ____fput+0x20/0x30
> task_work_run+0xc0/0xf0
> do_notify_resume+0x2b0/0x328
> work_pending+0x8/0x10
>
> The extra mapcount originated from pagefault handler, which handled
> pagefault for vma that has already been detached. vma is detached
> under mmap_sem write lock by detach_vmas_to_be_unmapped(), which
> also invalidates vmacache.
>
> When pagefault handler (under mmap_sem read lock) called find_vma(),
> vmacache_valid() wrongly reported vmacache as valid.
>
> After rwsem down_read() returns via 'queue empty' path (as of v5.2),
> it does so without issuing read_acquire on sem->count:
> down_read
> __down_read
> rwsem_down_read_failed
> __rwsem_down_read_failed_common
> raw_spin_lock_irq(&sem->wait_lock);
> if (list_empty(&sem->wait_list)) {
> if (atomic_long_read(&sem->count) >= 0) {
> raw_spin_unlock_irq(&sem->wait_lock);
> return sem;
>
> Suspected problem here is that last *_acquire on down_read() side
> happens before write side issues *_release:
> 1. writer: has the lock
> 2. reader: down_read() issues *read_acquire on entry
> 3. writer: mm->vmacache_seqnum++; downgrades lock (*fetch_add_release)
> 4. reader: __rwsem_down_read_failed_common() finds it can take lock and
> returns
> 5. reader: observes stale mm->vmacache_seqnum
>
> I can reproduce the problem by running LTP mtest06 in a loop and building
> kernel (-j $NCPUS) in parallel. It does reproduce since v4.20 up to v5.2
> on arm64 HPE Apollo 70 (224 CPUs, 256GB RAM, 2 nodes). It triggers reliably
> within ~hour. Patched kernel ran fine for 10+ hours with clean dmesg.
> Tests were done against v5.2, since commit cf69482d62d9 ("locking/rwsem:
> Enable readers spinning on writer") makes it much harder to reproduce.
>
> v2: Move barrier after test (Waiman Long)
> Use smp_acquire__after_ctrl_dep() (Peter Zijlstra)
>
> Related:
> https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/mem/mtest06/mmap1.c
> Related: commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in
> munmap")
> Fixes: 4b486b535c33 ("locking/rwsem: Exit read lock slowpath if queue empty &
> no writer")
>
> Signed-off-by: Jan Stancek <jstan...@redhat.com>
> Cc: sta...@vger.kernel.org # v4.20+
> Cc: Waiman Long <long...@redhat.com>
> Cc: Davidlohr Bueso <dbu...@suse.de>
> Cc: Will Deacon <w...@kernel.org>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Ingo Molnar <mi...@redhat.com>
> ---
> kernel/locking/rwsem.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 37524a47f002..5ac72b60608b 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -1032,6 +1032,7 @@ static inline bool rwsem_reader_phase_trylock(struct
> rw_semaphore *sem,
> */
> if (adjustment && !(atomic_long_read(&sem->count) &
> (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
> + smp_acquire__after_ctrl_dep();
> raw_spin_unlock_irq(&sem->wait_lock);
> rwsem_set_reader_owned(sem);
> lockevent_inc(rwsem_rlock_fast);
If you add a comment to the code outlining the issue (preferably as a litmus
test involving sem->count and some shared data which happens to be
vmacache_seqnum in your test)), then:
Reviewed-by: Will Deacon <w...@kernel.org>
Thanks,
Will
