From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f182.google.com (mail-pd0-f182.google.com [209.85.192.182]) by kanga.kvack.org (Postfix) with ESMTP id 6599C6B00C8 for ; Wed, 6 Nov 2013 05:34:37 -0500 (EST) Received: by mail-pd0-f182.google.com with SMTP id q10so9972509pdj.13 for ; Wed, 06 Nov 2013 02:34:36 -0800 (PST) Received: from psmtp.com ([74.125.245.155]) by mx.google.com with SMTP id qk4si16906846pac.235.2013.11.06.02.34.34 for ; Wed, 06 Nov 2013 02:34:35 -0800 (PST) Date: Wed, 6 Nov 2013 10:34:03 +0000 From: Will Deacon Subject: Re: [PATCH] mm: create a separate slab for page->ptl allocation Message-ID: <20131106103403.GB21074@mudshark.cambridge.arm.com> References: <1382442839-7458-1-git-send-email-kirill.shutemov@linux.intel.com> <20131105150145.734a5dd5b5d455800ebfa0d3@linux-foundation.org> <20131105224217.GC20167@shutemov.name> <20131105155619.021f32eba1ca8f15a73ed4c9@linux-foundation.org> <20131105231310.GE20167@shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131105231310.GE20167@shutemov.name> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Andrew Morton , "Kirill A. Shutemov" , Peter Zijlstra , Ingo Molnar , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "linux-arch@vger.kernel.org" On Tue, Nov 05, 2013 at 11:13:11PM +0000, Kirill A. Shutemov wrote: > On Tue, Nov 05, 2013 at 03:56:19PM -0800, Andrew Morton wrote: > > On Wed, 6 Nov 2013 00:42:17 +0200 "Kirill A. Shutemov" wrote: > > > > > > > #if USE_SPLIT_PTE_PTLOCKS > > > > > +struct kmem_cache *page_ptl_cachep; > > > > > +void __init ptlock_cache_init(void) > > > > > +{ > > > > > + if (sizeof(spinlock_t) > sizeof(long)) > > > > > + page_ptl_cachep = kmem_cache_create("page->ptl", > > > > > + sizeof(spinlock_t), 0, SLAB_PANIC, NULL); > > > > > +} > > > > > > > > Confused. If (sizeof(spinlock_t) > sizeof(long)) happens to be false > > > > then the kernel will later crash. It would be better to use BUILD_BUG_ON() > > > > here, if that works. Otherwise BUG_ON. > > > > > > if (sizeof(spinlock_t) > sizeof(long)) is false, we don't need dynamicly > > > allocate page->ptl. It's embedded to struct page itself. __ptlock_alloc() > > > never called in this case. > > > > OK. Please add a comment explaining this so the next reader doesn't get > > tripped up like I was. > > Okay, I will tomorrow. > > > Really the function shouldn't exist in this case. It is __init so the > > sin is not terrible, but can this be arranged? > > I would like to get rid of __ptlock_alloc()/__ptlock_free() too, but I > don't see a way within C: we need to know sizeof(spinlock_t) on > preprocessor stage. FWIW: if the architecture selects ARCH_USE_CMPXCHG_LOCKREF, then a spinlock_t is 32-bit (assuming that unsigned int is also 32-bit). Will -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org