On 7/17/19 9:13 AM, Will Deacon wrote: > 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
Agreed. A comment just above smp_acquire__after_ctrl_dep() on why this is needed will be great. Other than that, Acked-by: Waiman Long <long...@redhat.com>