Date:   Fri, 2 Sep 2022 11:09:08 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Vlastimil Babka <vbabka@...e.cz>, rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, rushikesh.s.kadam@...el.com,
        urezki@...il.com, neeraj.iitr10@...il.com, frederic@...nel.org,
        paulmck@...nel.org, rostedt@...dmis.org, vineeth@...byteword.org,
        boqun.feng@...il.com, Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH v5 02/18] mm/sl[au]b: rearrange struct slab fields to
 allow larger rcu_head



On 9/2/2022 5:30 AM, Vlastimil Babka wrote:
> On 9/2/22 11:26, Vlastimil Babka wrote:
>> On 9/2/22 00:17, Joel Fernandes (Google) wrote:
>>> From: Vlastimil Babka <vbabka@...e.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@...lfernandes.org>
>>> Signed-off-by: Vlastimil Babka <vbabka@...e.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@...il.com>
> 
> patch 2
> Acked-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
> 
> [1] https://lore.kernel.org/all/20220826090912.11292-1-vbabka@suse.cz/


Sorry for injecting confusion - my main intent with including the mm patches in
this series is to make it easier for other reviewers/testers to backport the
series to their kernels in one shot. Some reviewers expressed interested in
trying out the series.

I think it is best to let the -mm patches in the series go through the slab
tree, as you also have the Acks/Reviews there and will take sure those
dependencies are out of the way.

My lesson here is to be more clear, I could have added some notes for context
below the "---" of those mm patches.

Thanks again for your help,

- Joel

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