* SeongJae Park <sj@xxxxxxxxxx> [240904 14:53]: > On Wed, 4 Sep 2024 11:41:28 -0700 Guenter Roeck <linux@xxxxxxxxxxxx> wrote: > > > On 9/4/24 10:29, SeongJae Park wrote: > > > damon_test_three_regions_in_vmas() initializes a maple tree with > > > MM_MT_FLAGS. The flags contains MT_FLAGS_LOCK_EXTERN, which means > > > mt_lock of the maple tree will not be used. And therefore the maple > > > tree initialization code skips initialization of the mt_lock. However, > > > __link_vmas(), which adds vmas for test to the maple tree, uses the > > > mt_lock. In other words, the uninitialized spinlock is used. The > > > problem becomes clear when spinlock debugging is turned on, since it > > > reports spinlock bad magic bug. > > > > > > Fix the issue by excluding MT_FLAGS_LOCK_EXTERN from the maple tree > > > initialization flags. Note that we don't use empty flags to make it > > > further similar to the usage of mm maple tree, and to be prepared for > > > possible future changes, as suggested by Liam. > > > > > > Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx> > > > Closes: https://lore.kernel.org/1453b2b2-6119-4082-ad9e-f3c5239bf87e@xxxxxxxxxxxx > > > Fixes: d0cf3dd47f0d ("damon: convert __damon_va_three_regions to use the VMA iterator") > > > Suggested-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> > > > Signed-off-by: SeongJae Park <sj@xxxxxxxxxx> > > > --- > > > Changes from v1 > > > (https://lore.kernel.org/20240904004534.1189-1-sj@xxxxxxxxxx) > > > - Keep lock usage and update the initialization flags (Liam) > > > > Not sure I understand how this is better. Is the resulting rcu warning > > considered to be irrelevant or a separate problem ? > > I believe it's a separate potential problem (warning) that deserves a look. > Meanwhile, this patch is fixing an issue that is definitely a bug, so better to > fix right now, imho. > Whenever you walk the tree, you need to hold the write lock or the rcu read lock. I believe that __damon_va_three_regions() needs to have the rcu_read_lock()/ruc_read_unlock() around the for_each_vma() loop. I sent a patch to the old thread that I suspect will fix it - but I don't see the issue so it is hard to say for sure. Thanks, Liam
