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@xxxxxxx> > >>> > >>> 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@xxxxxxxxxxxxxxxxx/ > >>> > >>> Reported-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> > >>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > >> > >> 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@xxxxxxxxx> > > > > patch 2 > > Acked-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> > > > > [1] https://lore.kernel.org/all/20220826090912.11292-1-vbabka@xxxxxxx/ > > > 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
