On Wed, Sep 28, 2022 at 06:20:10PM +0200, Vlastimil Babka wrote: > On 9/28/22 15:48, Joel Fernandes wrote: > > 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(). > > When I tried that, the result wasn't really nice: > > https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@xxxxxxx/ > > And what if there's another conflicting page "type" later. Or the debugging > variant of rcu_head in struct page itself. The __PageMovable() is just too > fragile. > > > > > 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). :-) > > But we'll have to do that for now and postpone to 6.2 I'm afraid as merge > window for 6.1 is too close to have confidence in any solution that we came > up this moment. Agreed. > > > > 2. make __PageMovable() to use true page flag, with approach [2]) > > > > What are the drawbacks of making it a true flag? > > Even if we free PageSlab, I'm sure there will be better uses of a free page > flag than __PageMovable. > > 3. With frozen page allocation > https://lore.kernel.org/all/20220809171854.3725722-1-willy@xxxxxxxxxxxxx/ > > slab pages will have refcount 0 and compaction will skip them for that > reason. But it had other unresolved issues with page isolation code IIRC. > 4. Always align function rcu_free_slab() with 'aligned' attribute [1] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-aligned-function-attribute [2] __aligned in include/linux/compiler_attributes.h > > 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 > -- Thanks, Hyeonggon
