Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932420AbYARVDZ (ORCPT ); Fri, 18 Jan 2008 16:03:25 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1762664AbYARVDI (ORCPT ); Fri, 18 Jan 2008 16:03:08 -0500 Received: from wa-out-1112.google.com ([209.85.146.183]:22501 "EHLO wa-out-1112.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762133AbYARVDG (ORCPT ); Fri, 18 Jan 2008 16:03:06 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=message-id:date:from:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=riJKEfJRUpdwWII8hkCqRirT1XOTIG1oT/HIsoA32LqmANtBqZt3kHfcYa1d0u0p8g6ptN//e9pqihsJsFY9xVkskcNEi7jeLYsRFzkM/9HUVVV77u9ZsOYb0cEo5fa3/29awAdcop/5goXDyG7XGdQKRXFnjxShqQy1aeLCB50= Message-ID: <4df4ef0c0801181303o6656832g8b63d2a119a86a9c@mail.gmail.com> Date: Sat, 19 Jan 2008 00:03:04 +0300 From: "Anton Salikhmetov" To: "Linus Torvalds" Subject: Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files Cc: "Miklos Szeredi" , peterz@infradead.org, linux-mm@kvack.org, jakob@unthought.net, linux-kernel@vger.kernel.org, valdis.kletnieks@vt.edu, riel@redhat.com, ksm@42.dk, staubach@redhat.com, jesper.juhl@gmail.com, akpm@linux-foundation.org, protasnb@gmail.com, r.e.wolff@bitwizard.nl, hidave.darkstar@gmail.com, hch@infradead.org In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <12006091182260-git-send-email-salikhmetov@gmail.com> <4df4ef0c0801181158s3f783beaqead3d7049d4d3fa7@mail.gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2064 Lines: 58 2008/1/18, Linus Torvalds : > > > On Fri, 18 Jan 2008, Anton Salikhmetov wrote: > > > > The current solution doesn't hit the performance at all when compared to > > the competitor POSIX-compliant systems. It is faster and does even more > > than the POSIX standard requires. > > Your current patches have two problems: > - they are simply unnecessarily invasive for a relatively simple issue > - all versions I've looked at closer are buggy too > > Example: > > + if (pte_dirty(*pte) && pte_write(*pte)) > + *pte = pte_wrprotect(*pte); > > Uhhuh. Looks simple enough. Except it does a non-atomic pte access while > other CPU's may be accessing it and updating it from their hw page table > walkers. What will happen? Who knows? I can see lost access bits at a > minimum. > > IOW, this isn't simple code. It's code that it is simple to screw up. In > this case, you really need to use ptep_set_wrprotect(), for example. Before using pte_wrprotect() the vma_wrprotect() routine uses the pte_offset_map_lock() macro to get the PTE and to acquire the ptl spinlock. Why did you say that this code was not SMP-safe? It should be atomic, I think. > > So why not do it in many fewer lines with that simpler vma->dirty flag? Neither the dirty flag you suggest, nor the AS_MCTIME flag I've introduced in my previous solutions solve the following problem: - mmap() - a write reference - msync() with MS_ASYNC - a write reference - msync() with MS_ASYNC The POSIX standard requires the ctime and mtime stamps to be updated not later than at the second call to msync() with the MS_ASYNC flag. Some other POSIX-compliant operating system such as HP-UX and FreeBSD satisfy this POSIX requirement. Linux does not. > > Linus > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/