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 663726B0038 for ; Tue, 18 Nov 2014 18:21:12 -0500 (EST) Received: by mail-pd0-f182.google.com with SMTP id r10so40080pdi.41 for ; Tue, 18 Nov 2014 15:21:12 -0800 (PST) Received: from lgeamrelo01.lge.com (lgeamrelo01.lge.com. [156.147.1.125]) by mx.google.com with ESMTP id gz1si67876pbc.65.2014.11.18.15.21.08 for ; Tue, 18 Nov 2014 15:21:10 -0800 (PST) Date: Wed, 19 Nov 2014 08:21:39 +0900 From: Minchan Kim Subject: Re: [PATCH] zsmalloc: correct fragile [kmap|kunmap]_atomic use Message-ID: <20141118232139.GA7393@bbox> References: <1415927461-14220-1-git-send-email-minchan@kernel.org> <20141114150732.GA2402@cerebellum.variantweb.net> <20141118150138.668c81fda55c3ce39d7b2aac@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20141118150138.668c81fda55c3ce39d7b2aac@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Seth Jennings , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Nitin Gupta , Sergey Senozhatsky , Dan Streetman , Jerome Marchand Hello Andrew, On Tue, Nov 18, 2014 at 03:01:38PM -0800, Andrew Morton wrote: > On Fri, 14 Nov 2014 09:07:32 -0600 Seth Jennings wrote: > > > On Fri, Nov 14, 2014 at 10:11:01AM +0900, Minchan Kim wrote: > > > The kunmap_atomic should use virtual address getting by kmap_atomic. > > > However, some pieces of code in zsmalloc uses modified address, > > > not the one got by kmap_atomic for kunmap_atomic. > > > > > > It's okay for working because zsmalloc modifies the address > > > inner PAGE_SIZE bounday so it works with current kmap_atomic's > > > implementation. But it's still fragile with potential changing > > > of kmap_atomic so let's correct it. > > It is a bit alarming, but I've seen code elsewhere in which a modified > pointer is passed to kunmap_atomic(). So the kunmap_atomic() interface > is "kvaddr should point somewhere into the page" and that won't be > changing without a big effort. > > > Seems like you could just use PAGE_MASK to get the base page address > > from link like this: > > I think Minchan's approach is better: it explicitly retains the > kmap_atomic() return value for passing to kunmap_atomic(). That's > nicer than modifying it and then setting it back again. > > I mean, a cleaner way of implementing your suggestion would be > > void kunmap_atomic_unaligned(void *p) > { > kunmap_atomic(void *)((unsigned long)p & PAGE_MASK); > } > > but then one looks at > > void __kunmap_atomic(void *kvaddr) > { > unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK; > > and asks "what the heck". > > > So I dunno. We could leave the code as-is. I have no strong feelings > either way. Minchan's patch has no effect on zsmalloc.o section sizes > with my compiler. I hope to merge my patch. Main reason I sent the patch is I got a subtle bug when I implement new feature of zsmalloc(ie, compaction) due to link's mishandling (ie, link was over page boundary by my fault). Although it was totally my mistake, it took time for a while to find a root cause because unpredictable kmapped address should be unmapped so it's almost random crash. IOW, it's fragile to depend on kunmap_atomic's internal and at least, I wanted to make zram code more robust. Thanks. > > -- > 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 -- Kind regards, Minchan Kim -- 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