On Wed, Sep 28, 2022 at 02:49:02PM +0900, Hyeonggon Yoo wrote:
On Tue, Sep 27, 2022 at 10:16:35PM -0700, Hugh Dickins wrote:
It's a bug in linux-next, but taking me too long to identify which
commit is "to blame", so let me throw it over to you without more
delay: I think __PageMovable() now needs to check !PageSlab().
I had made a small experimental change somewhere, rebuilt and rebooted,
was not surprised to crash once swapping and compaction came in,
but was surprised to find the crash in isolate_movable_page(),
called by compaction's isolate_migratepages_block().
page->mapping was ffffffff811303aa, which qualifies as __PageMovable(),
which expects struct movable_operations at page->mapping minus low bits.
But ffffffff811303aa was the address of SLUB's rcu_free_slab(): I have
CONFIG_CC_OPTIMIZE_FOR_SIZE=y, so function addresses may have low bits set.
Over to you! Thanks,
Hugh
Wow, didn't expect this.
Thank you for report!
That should be due to commit 65505d1f2338e7
("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head")
as now rcu_head can use some bits that shares with mapping.
Hmm IMO we have two choices...
1. simply drop the commit as it's only for debugging (RCU folks may not like [1])
Yeah definitely don't like this option as patches are out that depend on
this (not yet merged though). :-)
2. make __PageMovable() to use true page flag, with approach [2])
What are the drawbacks of making it a true flag?
thanks,
- Joel
[1] https://lore.kernel.org/all/85afd876-d8bb-0804-b2c5-48ed3055e702@xxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/linux-mm/20220919125708.276864-1-42.hyeyoo@xxxxxxxxx/
Thanks!
--
Thanks,
Hyeonggon