Hi Vineet, Peter, all,
-----Original Message----- From: Vineet Gupta vgupta@synopsys.com Sent: Thursday, February 14, 2019 2:24 AM To: Peter Zijlstra peterz@infradead.org Cc: David Laight David.Laight@ACULAB.COM; Alexey Brodkin alexey.brodkin@synopsys.com; linux-snps- arc@lists.infradead.org; Arnd Bergmann arnd.bergmann@linaro.org; linux-kernel@vger.kernel.org; stable@vger.kernel.org; Mark Rutland mark.rutland@arm.com 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