On 9/29/22 23:54, 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@xxxxxxx/ >> >> >> >> 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. Yeah could have been more explicit about that, sorry. But made the whole thing a very last merge so I can still drop it before the pull request. > 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(). Right. > Regarding (b): I booted the PowerMac G5 to take a look, and dredged up > the relevant phrase "function descriptor" from depths of my memory: I > was right to consider that case, but it's not a worry - the first field > of a function descriptor structure (on all the architectures I found it) > is the function address, so the function descriptor address would be > aligned 4 or 8 anyway. Thanks. I admit I wasn't that thorough, just consulted somebody internally :) > Regarding "conflicting" alignment requests: yes, I agree with you, > it would have to be a toolchain bug if when asked to align 2 and to > align 4, it chose not to align 4. Yeah. But I still would be less worried if another __aligned(X) function existed in the tree already. Found only data. I assume the i915 thing wasn't fixed like this in the tree? So if there are buggy toolchains or anything, it will be us to discover them. So I think we still should defuse the __PageMovable() mine somehow. > So, no worries at my end now. > Hugh
