From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 22 Aug 2007 11:04:22 -0700 From: Andrew Morton Subject: Re: [RFC 3/3] SGI Altix cross partition memory (XPMEM) Message-Id: <20070822110422.65c990e5.akpm@linux-foundation.org> In-Reply-To: <20070822170011.GA20155@sgi.com> References: <20070810010659.GA25427@sgi.com> <20070810011435.GD25427@sgi.com> <20070809231542.f6dcce8c.akpm@linux-foundation.org> <20070822170011.GA20155@sgi.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Dean Nelson Cc: linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, tony.luck@intel.com, jes@sgi.com List-ID: On Wed, 22 Aug 2007 12:00:11 -0500 Dean Nelson wrote: > > 3) WARNING: declaring multiple variables together should be avoided > > checkpatch.pl is erroneously commplaining about the following found in five > different functions in arch/ia64/sn/kernel/xpmem_pfn.c. > > int n_pgs = xpmem_num_of_pages(vaddr, size); What warning does it generate here? > > - xpmem_fault_handler() appears to have imposed a kernel-wide rule that > > when taking multiple mmap_sems, one should take the lowest-addressed one > > first? If so, that probably wants a mention in that locking comment in > > filemap.c > > Sure. After looking at the lock ordering comment block in mm/filemap.c, it > wasn't clear to me how best to document this. Any suggestions/help would > be most appreciated. umm, * when taking multiple mmap_sems, one should take the lowest-addressed one * first ;) > > - xpmem_fault_handler() does atomic_dec(&seg_tg->mm->mm_users). What > > happens if that was the last reference? > > When /dev/xpmem is opened by a user process, xpmem_open() incs mm_users > and when it is flushed, xpmem_flush() decs it (via mmput()) after having > ensured that no XPMEM attachments exist of this mm. Thus the dec in > xpmem_fault_handler() will never take it to 0. OK. Generally if a reviewer asks a question like this, it indicates that a code comment is needed. Because it is likely that others will later wonder the same thing. > > - Has it all been tested with lockdep enabled? Jugding from all the use > > of SPIN_LOCK_UNLOCKED, it has not. > > > > Oh, ia64 doesn't implement lockdep. For this code, that is deeply > > regrettable. > > No, it hasn't been tested with lockdep. But I have switched it from using > SPIN_LOCK_UNLOCKED to spin_lock_init(). > > > ! This code all predates the nopage->fault conversion and won't work in > > current kernels. > > I've switched from using nopage to using fault. I read that it is intended > that nopfn also goes away. If this is the case, then the BUG_ON if VM_PFNMAP > is set would make __do_fault() a rather unfriendly replacement for do_no_pfn(). > > > - xpmem_attach() does smp_processor_id() in preemptible code. Lucky that > > ia64 doesn't do preempt? > > Actually, the code is fine as is even with preemption configured on. All it's > doing is ensuring that the thread was previously pinned to the CPU it's > currently running on. If it is, it can't be moved to another CPU via > preemption, and if it isn't, the check will fail and we'll return -EINVAL > and all is well. OK. Running smp_processor_id() from within preemptible code will generate a warning, but the code is sneaky enough to prevent that warning if the calling task happens to be pinned to a single CPU. -- 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