| Message ID | 1483479365-13607-1-git-send-email-ross.zwisler@linux.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Tue 03-01-17 14:36:05, Ross Zwisler wrote: > Currently in DAX if we have three read faults on the same hole address we > can end up with the following: > > Thread 0 Thread 1 Thread 2 > -------- -------- -------- > dax_iomap_fault > grab_mapping_entry > lock_slot > <locks empty DAX entry> > > dax_iomap_fault > grab_mapping_entry > get_unlocked_mapping_entry > <sleeps on empty DAX entry> > > dax_iomap_fault > grab_mapping_entry > get_unlocked_mapping_entry > <sleeps on empty DAX entry> > dax_load_hole > find_or_create_page > ... > page_cache_tree_insert > dax_wake_mapping_entry_waiter > <wakes one sleeper> > __radix_tree_replace > <swaps empty DAX entry with 4k zero page> > > <wakes> > get_page > lock_page > ... > put_locked_mapping_entry > unlock_page > put_page > > <sleeps forever on the DAX > wait queue> > > The crux of the problem is that once we insert a 4k zero page, all locking > from then on is done in terms of that 4k zero page and any additional > threads sleeping on the empty DAX entry will never be woken. Fix this by > waking all sleepers when we replace the DAX radix tree entry with a 4k zero > page. This will allow all sleeping threads to successfully transition from > locking based on the DAX empty entry to locking on the 4k zero page. > > With the test case reported by Xiong this happens very regularly in my test > setup, with some runs resulting in 9+ threads in this deadlocked state. > With this fix I've been able to run that same test dozens of times in a > loop without issue. > > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> > Reported-by: Xiong Zhou <xzhou@redhat.com> > Fixes: commit ac401cc78242 ("dax: New fault locking") > Cc: Jan Kara <jack@suse.cz> > Cc: stable@vger.kernel.org # 4.7+ Ah, very good catch. You can add: Reviewed-by: Jan Kara <jack@suse.cz> I wonder why I was not able to reproduce this... Probably the timing didn't work out right on my test machine. Honza > --- > > This issue exists as far back as v4.7, and I was easly able to reproduce it > with v4.7 using the same test. > > Unfortunately this patch won't apply cleanly to the stable trees, but the > change is very simple and should be easy to replicate by hand. Please ping > me if you'd like patches that apply cleanly to the v4.9 and v4.8.15 trees. > > --- > mm/filemap.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/filemap.c b/mm/filemap.c > index d0e4d10..b772a33 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -138,7 +138,7 @@ static int page_cache_tree_insert(struct address_space *mapping, > dax_radix_locked_entry(0, RADIX_DAX_EMPTY)); > /* Wakeup waiters for exceptional entry lock */ > dax_wake_mapping_entry_waiter(mapping, page->index, p, > - false); > + true); > } > } > __radix_tree_replace(&mapping->page_tree, node, slot, page, > -- > 2.7.4 >
On Tue, Jan 03, 2017 at 02:36:05PM -0700, Ross Zwisler wrote: > Currently in DAX if we have three read faults on the same hole address we > can end up with the following: > > Thread 0 Thread 1 Thread 2 > -------- -------- -------- > dax_iomap_fault > grab_mapping_entry > lock_slot > <locks empty DAX entry> > > dax_iomap_fault > grab_mapping_entry > get_unlocked_mapping_entry > <sleeps on empty DAX entry> > > dax_iomap_fault > grab_mapping_entry > get_unlocked_mapping_entry > <sleeps on empty DAX entry> > dax_load_hole > find_or_create_page > ... > page_cache_tree_insert > dax_wake_mapping_entry_waiter > <wakes one sleeper> > __radix_tree_replace > <swaps empty DAX entry with 4k zero page> > > <wakes> > get_page > lock_page > ... > put_locked_mapping_entry > unlock_page > put_page > > <sleeps forever on the DAX > wait queue> > > The crux of the problem is that once we insert a 4k zero page, all locking > from then on is done in terms of that 4k zero page and any additional > threads sleeping on the empty DAX entry will never be woken. Fix this by > waking all sleepers when we replace the DAX radix tree entry with a 4k zero > page. This will allow all sleeping threads to successfully transition from > locking based on the DAX empty entry to locking on the 4k zero page. > > With the test case reported by Xiong this happens very regularly in my test > setup, with some runs resulting in 9+ threads in this deadlocked state. > With this fix I've been able to run that same test dozens of times in a > loop without issue. > > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> > Reported-by: Xiong Zhou <xzhou@redhat.com> > Fixes: commit ac401cc78242 ("dax: New fault locking") > Cc: Jan Kara <jack@suse.cz> > Cc: stable@vger.kernel.org # 4.7+ > --- Positive test result of this patch for this issue and the regression tests. Great job! > > This issue exists as far back as v4.7, and I was easly able to reproduce it > with v4.7 using the same test. > > Unfortunately this patch won't apply cleanly to the stable trees, but the > change is very simple and should be easy to replicate by hand. Please ping > me if you'd like patches that apply cleanly to the v4.9 and v4.8.15 trees. > > --- > mm/filemap.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/filemap.c b/mm/filemap.c > index d0e4d10..b772a33 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -138,7 +138,7 @@ static int page_cache_tree_insert(struct address_space *mapping, > dax_radix_locked_entry(0, RADIX_DAX_EMPTY)); > /* Wakeup waiters for exceptional entry lock */ > dax_wake_mapping_entry_waiter(mapping, page->index, p, > - false); > + true); > } > } > __radix_tree_replace(&mapping->page_tree, node, slot, page, > -- > 2.7.4 >
diff --git a/mm/filemap.c b/mm/filemap.c index d0e4d10..b772a33 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -138,7 +138,7 @@ static int page_cache_tree_insert(struct address_space *mapping, dax_radix_locked_entry(0, RADIX_DAX_EMPTY)); /* Wakeup waiters for exceptional entry lock */ dax_wake_mapping_entry_waiter(mapping, page->index, p, - false); + true); } } __radix_tree_replace(&mapping->page_tree, node, slot, page,
Currently in DAX if we have three read faults on the same hole address we can end up with the following: Thread 0 Thread 1 Thread 2 -------- -------- -------- dax_iomap_fault grab_mapping_entry lock_slot <locks empty DAX entry> dax_iomap_fault grab_mapping_entry get_unlocked_mapping_entry <sleeps on empty DAX entry> dax_iomap_fault grab_mapping_entry get_unlocked_mapping_entry <sleeps on empty DAX entry> dax_load_hole find_or_create_page ... page_cache_tree_insert dax_wake_mapping_entry_waiter <wakes one sleeper> __radix_tree_replace <swaps empty DAX entry with 4k zero page> <wakes> get_page lock_page ... put_locked_mapping_entry unlock_page put_page <sleeps forever on the DAX wait queue> The crux of the problem is that once we insert a 4k zero page, all locking from then on is done in terms of that 4k zero page and any additional threads sleeping on the empty DAX entry will never be woken. Fix this by waking all sleepers when we replace the DAX radix tree entry with a 4k zero page. This will allow all sleeping threads to successfully transition from locking based on the DAX empty entry to locking on the 4k zero page. With the test case reported by Xiong this happens very regularly in my test setup, with some runs resulting in 9+ threads in this deadlocked state. With this fix I've been able to run that same test dozens of times in a loop without issue. Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Reported-by: Xiong Zhou <xzhou@redhat.com> Fixes: commit ac401cc78242 ("dax: New fault locking") Cc: Jan Kara <jack@suse.cz> Cc: stable@vger.kernel.org # 4.7+ --- This issue exists as far back as v4.7, and I was easly able to reproduce it with v4.7 using the same test. Unfortunately this patch won't apply cleanly to the stable trees, but the change is very simple and should be easy to replicate by hand. Please ping me if you'd like patches that apply cleanly to the v4.9 and v4.8.15 trees. --- mm/filemap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)