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Message-ID: <YzkmErHFyYW3awRn@hyeyoo> Date: Sun, 2 Oct 2022 14:48:02 +0900 From: Hyeonggon Yoo <42.hyeyoo@...il.com> To: Hugh Dickins <hughd@...gle.com> Cc: Vlastimil Babka <vbabka@...e.cz>, David Laight <David.Laight@...lab.com>, Joel Fernandes <joel@...lfernandes.org>, Matthew Wilcox <willy@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, rcu@...r.kernel.org Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE On Thu, Sep 29, 2022 at 02:54:45PM -0700, Hugh Dickins wrote: > On Thu, 29 Sep 2022, Vlastimil Babka wrote: > > On 9/28/22 19:50, Hugh Dickins wrote: > > > On Wed, 28 Sep 2022, Vlastimil Babka wrote: > > >> On 9/28/22 15:48, Joel Fernandes wrote: > > >> > On Wed, Sep 28, 2022 at 02:49:02PM +0900, Hyeonggon Yoo wrote: > > >> >> On Tue, Sep 27, 2022 at 10:16:35PM -0700, Hugh Dickins wrote: > > >> >>> It's a bug in linux-next, but taking me too long to identify which > > >> >>> commit is "to blame", so let me throw it over to you without more > > >> >>> delay: I think __PageMovable() now needs to check !PageSlab(). > > >> > > >> When I tried that, the result wasn't really nice: > > >> > > >> https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@suse.cz/ > > >> > > >> And what if there's another conflicting page "type" later. Or the debugging > > >> variant of rcu_head in struct page itself. The __PageMovable() is just too > > >> fragile. > > > > > > I don't disagree (and don't really know all the things you're thinking > > > of in there). But if it's important to rescue this feature for 6.1, a > > > different approach may be the very simple patch below (I met a similar > > > issue with OPTIMIZE_FOR_SIZE in i915 a year ago, and just remembered). > > > > > > But you be the judge of it: (a) I do not know whether rcu_free_slab > > > is the only risky address ever stuffed into that field; and (b) I'm > > > clueless when it comes to those architectures (powerpc etc) where the > > > the address of a function is something different from the address of > > > the function (have I conveyed my cluelessness adequately?). > > > > Thanks a lot Hugh! That's a sufficiently small fix (compared to the other > > options) that I'm probably give it one last try. > > I suddenly worried that you might be waiting on me for a Signed-off-by, > which I couldn't give until I researched my reservations (a) and (b): > but I'm pleased to see from your kernel.org tree that you've gone ahead > and folded it in - thanks. > > Regarding (a): great, you've found it too, mm/slab.c's kmem_rcu_free() > looks like it needs the same __aligned(4) as mm/slub.c's rcu_free_slabi(). 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)? -- Thanks, Hyeonggon
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