Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

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@suse.cz/
> 
> 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@infradead.org/
> 
> 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@joelfernandes.org/
> > > [2] https://lore.kernel.org/linux-mm/20220919125708.276864-1-42.hyeyoo@gmail.com/
> > > 
> > > Thanks!
> > > 
> > > -- 
> > > Thanks,
> > > Hyeonggon
> 

-- 
Thanks,
Hyeonggon