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Date:   Mon, 24 Oct 2022 18:49:16 +0200
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Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE
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From:   Vlastimil Babka <vbabka@suse.cz>
To:     Matthew Wilcox <willy@infradead.org>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     Hugh Dickins <hughd@google.com>,
        David Laight <David.Laight@aculab.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        rcu@vger.kernel.org
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On 10/24/22 16:35, Vlastimil Babka wrote:
> On 10/3/22 19:00, Matthew Wilcox wrote:
>> On Sun, Oct 02, 2022 at 02:48:02PM +0900, Hyeonggon Yoo wrote:
>>> Just one more thing, rcu_leak_callback too. RCU seem to use it
>>> internally to catch double call_rcu().
>>> 
>>> And some suggestions:
>>> - what about adding runtime WARN() on slab init code to catch
>>>   unexpected arch/toolchain issues?
>>> - instead of 4, we may use macro definition? like (PAGE_MAPPING_FLAGS + 1)?
>> 
>> I think the real problem here is that isolate_movable_page() is
>> insufficiently paranoid.  Looking at the gyrations that GUP and the
>> page cache do to convince themselves that the page they got really is
>> the page they wanted, there are a few missing pieces (eg checking that
>> you actually got a refcount on _this_ page and not some random other
>> page you were temporarily part of a compound page with).
>> 
>> This patch does three things:
>> 
>>  - Turns one of the comments into English.  There are some others
>>    which I'm still scratching my head over.
>>  - Uses a folio to help distinguish which operations are being done
>>    to the head vs the specific page (this is somewhat an abuse of the
>>    folio concept, but it's acceptable)
>>  - Add the aforementioned check that we're actually operating on the
>>    page that we think we want to be.
>>  - Add a check that the folio isn't secretly a slab.
>> 
>> We could put the slab check in PageMapping and call it after taking
>> the folio lock, but that seems pointless.  It's the acquisition of
>> the refcount which stabilises the slab flag, not holding the lock.
>> 
> 
> I would like to have a working safe version in -next, even if we are able
> simplify it later thanks to frozen refcounts. I've made a formal patch of
> yours, but I'm still convinced the slab check needs to be more paranoid so
> it can't observe a false positive __folio_test_movable() while missing the
> folio_test_slab(), hence I added the barriers as in my previous attempt [1].
> Does that work for you and can I add your S-o-b?

Tentatively the series is here for anyone interested, will send it for
proper review after the S-o-b is clarified.

https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-6.2/fit_rcu_head

> [1] https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@suse.cz/