Messages in this thread | | | Date | Wed, 18 Jun 2014 21:33:29 +0200 | From | "Luis R. Rodriguez" <> | Subject | Re: [RFT 1/2] printk: make dynamic kernel ring buffer alignemnt explicit |
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On Wed, Jun 18, 2014 at 09:56:03AM -0600, Stephen Warren wrote: > On 06/18/2014 05:14 AM, Luis R. Rodriguez wrote: > > From: "Luis R. Rodriguez" <mcgrof@suse.com> > > > > We have to consider alignment for the ring buffer both for the > > default static size, and then also for when an dynamic allocation > > is made when the log_buf_len=n kernel parameter is passed to set > > the size specifically to a size larger than the default size set > > by the architecture through CONFIG_LOG_BUF_SHIFT. > > > > The default static kernel ring buffer can be aligned properly if > > architectures set CONFIG_LOG_BUF_SHIFT properly, we provide ranges > > for the size though so even if CONFIG_LOG_BUF_SHIFT has a sensible > > aligned value it can be reduced to a non aligned value. Commit > > 6ebb017de9 by Andrew ensures the static buffer is always aligned > > and the decision of alignment is done by the compiler by using > > __alignof__(struct log) (curious what value caused the crash?). > > IIRC the issue was that __log_buf's type is char[] so without the > __aligned it could have any alignment at all, e.g. 1 or 2. However, > struct printk_log is stored in the buffer rather than just char*, and so > if __log_buf isn't aligned to the required alignment for that structure, > that can caused unaligned accesses to fields in the structure, which > isn't supported on ARM in at least some cases. > > As such, I think the change to setup_log_buf() in this patch makes sense > (although I suppose in practice memblock_virt_alloc() probably has some > minimum internal alignment that dwards LOG_ALIGN, but that's an > implementation detail we shouldn't rely on).
Thanks for the feedback.
memblock_virt_alloc() will by default align to L1 cache, so if that satisfies the architecture alignment it should be safe, but perhaps not optimal for saving a few bytes. Still curious if without this patch a crash can be triggered somehow with some log_buf_len=n, if so this can go to stable.
Luis
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