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Date:   Fri, 2 Sep 2022 11:30:20 +0200
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Subject: Re: [PATCH v5 02/18] mm/sl[au]b: rearrange struct slab fields to
 allow larger rcu_head
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From:   Vlastimil Babka <vbabka@suse.cz>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com,
        urezki@gmail.com, neeraj.iitr10@gmail.com, frederic@kernel.org,
        paulmck@kernel.org, rostedt@goodmis.org, vineeth@bitbyteword.org,
        boqun.feng@gmail.com, Hyeonggon Yoo <42.hyeyoo@gmail.com>
References: <20220901221720.1105021-1-joel@joelfernandes.org>
 <20220901221720.1105021-3-joel@joelfernandes.org>
 <e84c90f2-d76f-83d4-d40b-403f894eda33@suse.cz>
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On 9/2/22 11:26, Vlastimil Babka wrote:
> On 9/2/22 00:17, Joel Fernandes (Google) wrote:
>> From: Vlastimil Babka <vbabka@suse.cz>
>> 
>> Joel reports [1] that increasing the rcu_head size for debugging
>> purposes used to work before struct slab was split from struct page, but
>> now runs into the various SLAB_MATCH() sanity checks of the layout.
>> 
>> This is because the rcu_head in struct page is in union with large
>> sub-structures and has space to grow without exceeding their size, while
>> in struct slab (for SLAB and SLUB) it's in union only with a list_head.
>> 
>> On closer inspection (and after the previous patch) we can put all
>> fields except slab_cache to a union with rcu_head, as slab_cache is
>> sufficient for the rcu freeing callbacks to work and the rest can be
>> overwritten by rcu_head without causing issues.
>> 
>> This is only somewhat complicated by the need to keep SLUB's
>> freelist+counters aligned for cmpxchg_double. As a result the fields
>> need to be reordered so that slab_cache is first (after page flags) and
>> the union with rcu_head follows. For consistency, do that for SLAB as
>> well, although not necessary there.
>> 
>> As a result, the rcu_head field in struct page and struct slab is no
>> longer at the same offset, but that doesn't matter as there is no
>> casting that would rely on that in the slab freeing callbacks, so we can
>> just drop the respective SLAB_MATCH() check.
>> 
>> Also we need to update the SLAB_MATCH() for compound_head to reflect the
>> new ordering.
>> 
>> While at it, also add a static_assert to check the alignment needed for
>> cmpxchg_double so mistakes are found sooner than a runtime GPF.
>> 
>> [1] https://lore.kernel.org/all/85afd876-d8bb-0804-b2c5-48ed3055e702@joelfernandes.org/
>> 
>> Reported-by: Joel Fernandes <joel@joelfernandes.org>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> 
> I've added patches 01 and 02 to slab tree for -next exposure before Joel's
> full series posting, but it should be also ok if rcu tree carries them with
> the whole patchset. I can then drop them from slab tree (there are no
> dependencies with other stuff there) so we don't introduce duplicite commits
> needlessly, just give me a heads up.

Ah but in that case please apply the reviews from my posting [1]

patch 1:
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

patch 2
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

[1] https://lore.kernel.org/all/20220826090912.11292-1-vbabka@suse.cz/