Hi Vineet, Peter, all, > -----Original Message----- > From: Vineet Gupta <vgupta@xxxxxxxxxxxx> > Sent: Thursday, February 14, 2019 2:24 AM > To: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: David Laight <David.Laight@xxxxxxxxxx>; Alexey Brodkin <alexey.brodkin@xxxxxxxxxxxx>; linux-snps- > arc@xxxxxxxxxxxxxxxxxxx; Arnd Bergmann <arnd.bergmann@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; > stable@xxxxxxxxxxxxxxx; Mark Rutland <mark.rutland@xxxxxxx> > Subject: Re: [PATCH] ARC: Explicitly set ARCH_SLAB_MINALIGN = 8 > > On 2/13/19 4:56 AM, Peter Zijlstra wrote: > > > > Personally I think u64 and company should already force natural > > alignment; but alas. > > But there is an ISA/ABI angle here too. e.g. On 32-bit ARC, LDD (load double) is > allowed to take a 32-bit aligned address to load a register pair. Thus all u64 > need not be 64-bit aligned (unless attribute aligned 8 etc) hence the relaxation > in ABI (alignment of long long is 4). You could certainly argue that we end up > undoing some of it anyways by defining things like ARCH_KMALLOC_MINALIGN to 8, but > still... > > > I though that was part of the reason we have __u64 > > and co., so that ABI is invariant to kernel alignment changes. > > Apparently not. > > >>> I suspect the slab allocator should be returning 8 byte aligned addresses > >>> on all systems.... > >> > >> why ? As I understand it is still not fool proof against the expected alignment of > >> inner members. There ought to be a better way to enforce all this. > > > > I agree that for ARC ARCH_SLAB_MINALIGN should be at least 8. > > This issue aside, are there other reasons ? Because making it 8 on ARC is just > pending the eventuality for later. But that's pretty much the same for other 32-bit arches that have 64-bit atomics like ARM etc. From what I may see from ARM's documentation for LDREXD/SRREXD they require double-word alignment of data as well. That said if for some reason atomic64_t variable is unaligned execution on any (or at least most) 32-bit architectures will lead to run-time failure, i.e. we'll know about it and this will be fixed. And what I'm doing by that change (ARCH_SLAB_MINALIGN=8 for ARC) I'm just working-around peculiarity of ARC ABI. Out of curiosity I checked if there're any other occurrences of "alingof(long long)" and there seems to be a couple of more: ----------------------------------->8----------------------------- # git grep alignof | grep "long long" ... kernel/workqueue.c:5693: WARN_ON(__alignof__(struct pool_workqueue) < __alignof__(long long)); mm/slab.c:155:#define REDZONE_ALIGN max(BYTES_PER_WORD, __alignof__(unsigned long long)) mm/slab.c:2034: if (ralign > __alignof__(unsigned long long)) ----------------------------------->8----------------------------- Not really sure how important is "kernel/workqueue.c" part but in case of "mm/slab.c" shouldn't we use ARCH_SLAB_MINALIGN there instead of that "not very meaningful" __alignof__(long long)? -Alexey
