From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with SMTP id F20676B00A1 for ; Mon, 18 Jan 2010 20:33:07 -0500 (EST) Date: Tue, 19 Jan 2010 09:33:03 +0800 From: Wu Fengguang Subject: Re: [PATCH 5/8] vmalloc: simplify vread()/vwrite() Message-ID: <20100119013303.GA12513@localhost> References: <20100113135305.013124116@intel.com> <20100113135957.833222772@intel.com> <20100114124526.GB7518@laptop> <20100118133512.GC721@localhost> <20100118142359.GA14472@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100118142359.GA14472@laptop> Sender: owner-linux-mm@kvack.org To: Nick Piggin Cc: Andrew Morton , LKML , Tejun Heo , Ingo Molnar , Andi Kleen , Hugh Dickins , Christoph Lameter , KAMEZAWA Hiroyuki , Linux Memory Management List List-ID: On Mon, Jan 18, 2010 at 07:23:59AM -0700, Nick Piggin wrote: > On Mon, Jan 18, 2010 at 09:35:12PM +0800, Wu Fengguang wrote: > > On Thu, Jan 14, 2010 at 05:45:26AM -0700, Nick Piggin wrote: > > > On Wed, Jan 13, 2010 at 09:53:10PM +0800, Wu Fengguang wrote: > > > > vread()/vwrite() is only called from kcore/kmem to access one page at a time. > > > > So the logic can be vastly simplified. > > > > > > > > The changes are: > > > > - remove the vmlist walk and rely solely on vmalloc_to_page() > > > > - replace the VM_IOREMAP check with (page && page_is_ram(pfn)) > > > > - rename to vread_page()/vwrite_page() > > > > > > > > The page_is_ram() check is necessary because kmap_atomic() is not > > > > designed to work with non-RAM pages. > > > > > > I don't know if you can really do this. Previously vmlist_lock would be > > > taken, which will prevent these vm areas from being freed. > > > > > > > Note that even for a RAM page, we don't own the page, and cannot assume > > > > it's a _PAGE_CACHE_WB page. > > > > > > So why is this not a problem for your patch? I don't see how you handle > > > it. > > > > Sorry I didn't handle it. Just hope to catch attentions from someone > > (ie. you :). > > > > It's not a problem for x86_64 at all. For others I wonder if any > > driver will vmalloc HIGHMEM pages with !_PAGE_CACHE_WB attribute.. > > > > So I noted the possible problem and leave it alone. > > Well it doesn't need to be vmalloc. Any kind of vmap like ioremap. And > these can be accompanied by changing the caching attribute. Like agp > code, for an example. But I don't know if that ever becomes a problem > in practice. Yes vmap in general can change caching attribute. However I only care about vmap that maps RAM pages, since my patch treats non-RAM pages as hole and won't access them. > > > What's the problem with the current code, exactly? I would prefer that > > > > - unnecessary complexity to handle multi-page case, since it's always > > called to access one single page; > > Fair point there. It just wasn't clear what exactly is your rationale > because this was in a set of other patches. > > > - the kmap_atomic() cache consistency problem, which I expressed some > > concern (without further action) > > Which kmap_atomic problem? Can you explain again? Virtual cache aliasing > problem you mean? Or caching attribute conflicts? kmap_atomic() assumes you own the page and always use _PAGE_CACHE_WB. So there may be conflicts if the page was !_PAGE_CACHE_WB. > The whole thing looks stupid though, apparently kmap is used to avoid "the > lock". But the lock is already held. We should just use the vmap > address. Yes. I wonder why Kame introduced kmap_atomic() in d0107eb07 -- given that he at the same time fixed the order of removing vm_struct and vmap in dd32c279983b. > > > you continue using the same vmlist locking and checking for validating > > > addresses. > > > > It's a reasonable suggestion. Kame, would you agree on killing the > > kmap_atomic() and revert to the vmlist walk? > > Yes, vmlist locking is always required to have a pin on the pages, and > IMO it should be quite easy to check for IOREMAP, so we should leave > that check there to avoid the possibility of regressions. I have no problem if Kame could dismiss my question :) Thanks, Fengguang -- 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