From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Subject: Re: O_DIRECT patch for processors with VIPT cache for mainline kernel (specifically arm in our case) Date: Thu, 20 Nov 2008 10:40:54 -0700 Message-ID: <20081120174054.GU1617@parisc-linux.org> References: <200811201759.01039.nickpiggin@yahoo.com.au> <200811210025.39568.nickpiggin@yahoo.com.au> <20081120135558.GB24308@linux-mips.org> <20081120152719.GS1617@parisc-linux.org> <20081120171713.GB27398@linux-mips.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Nick Piggin , Dmitry Adamushko , Russell King - ARM Linux , linux-fsdevel@vger.kernel.org, Naval Saini , linux-arch@vger.kernel.org, linux-arm-kernel@lists.arm.linux.org.uk, linux-kernel@vger.kernel.org, naval.saini@nxp.com, James Bottomley To: Ralf Baechle Return-path: Content-Disposition: inline In-Reply-To: <20081120171713.GB27398@linux-mips.org> Sender: linux-arch-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Thu, Nov 20, 2008 at 05:17:14PM +0000, Ralf Baechle wrote: > On Thu, Nov 20, 2008 at 08:27:19AM -0700, Matthew Wilcox wrote: > > I'm not quite sure why you need kmap_coherent(). If a page is mapped into > > userspace, you can find what address it's mapped to from > > page->mapping->i_mmap and page->index. OTOH, that's potentially > > Even if we know the userspace address of a page we do not necessarily have > a usable mapping for kernel purposes. The userspace mapping might be r/o > when we need r/w or it might be in another process. kmap_coherent takes > the job of creating a r/w mapping on a suitable kernel virtual address > that will avoid any aliases. Ah, I didn't do a good enough job of explaining. My question was why you needed the 'address' argument to kmap_coherent (and thus had a different prototype from kmap) instead of just implementing kmap() on mips. > The original reason for the introduction of kmap_coherent was avoiding > a cache alias in when a multi-threaded process forks. The issue has been > debated on lkml in 2006 as part of my submission of a patchset under the > subject of "Fix COW D-cache aliasing on fork". The description is somewhat > lengthy so I omit it here. > > One of the ugly parts of kmap_coherent() is that it cannot be used safely > if the page has been marked as dirty by flush_dcache_page(); the callers > know about this and deal with it. If you fix kmap() to only give you addresses that are coherent with userspace, you no longer need flush_dcache_page(). -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step."