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Date: Wed, 18 Jun 2014 13:46:18 -0600 From: Stephen Warren <swarren@...dotorg.org> To: "Luis R. Rodriguez" <mcgrof@...e.com> CC: "Luis R. Rodriguez" <mcgrof@...not-panic.com>, hpa@...ux.intel.com, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, Andrew Lunn <andrew@...n.ch>, Michal Hocko <mhocko@...e.cz>, Petr Mladek <pmladek@...e.cz>, Joe Perches <joe@...ches.com>, Arun KS <arunks.linux@...il.com>, Kees Cook <keescook@...omium.org>, Davidlohr Bueso <davidlohr@...com>, Chris Metcalf <cmetcalf@...era.com> Subject: Re: [RFT 1/2] printk: make dynamic kernel ring buffer alignemnt explicit On 06/18/2014 01:33 PM, Luis R. Rodriguez wrote: > 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@...e.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. If memblock_virt_alloc() aligns to L1 cache, then I believe that the crash would never trigger. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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