linux-kernel - Re: [PATCH v5 02/18] mm/sl[au]b: rearrange struct slab fields to allow larger rcu

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Date:   Sat, 3 Sep 2022 06:53:10 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, rushikesh.s.kadam@...el.com,
        urezki@...il.com, neeraj.iitr10@...il.com, frederic@...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 Fri, Sep 02, 2022 at 11:09:08AM -0400, Joel Fernandes wrote:
> 
> 
> 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,

Hello, Vlastimil, and thank you for putting these together!

I believe that your two patches should go in via the slab tree.
I am queueing them in -rcu only temporarily and just for convenience
in testing.  I expect that I will rebase them so that I can let your
versions cover things in -next.

							Thanx, Paul