From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f198.google.com (mail-qk0-f198.google.com [209.85.220.198]) by kanga.kvack.org (Postfix) with ESMTP id E3BD86B0253 for ; Thu, 13 Oct 2016 20:24:46 -0400 (EDT) Received: by mail-qk0-f198.google.com with SMTP id f128so61526431qkb.1 for ; Thu, 13 Oct 2016 17:24:46 -0700 (PDT) Received: from sender153-mail.zoho.com (sender153-mail.zoho.com. [74.201.84.153]) by mx.google.com with ESMTPS id x64si8122601qkb.136.2016.10.13.17.24.45 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Thu, 13 Oct 2016 17:24:46 -0700 (PDT) Subject: Re: [RFC PATCH 1/1] mm/percpu.c: fix memory leakage issue when allocate a odd alignment area References: <20161013233139.GE32534@mtj.duckdns.org> From: zijun_hu Message-ID: Date: Fri, 14 Oct 2016 08:23:06 +0800 MIME-Version: 1.0 In-Reply-To: <20161013233139.GE32534@mtj.duckdns.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Tejun Heo Cc: zijun_hu@htc.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, cl@linux.com On 2016/10/14 7:31, Tejun Heo wrote: > On Tue, Oct 11, 2016 at 09:24:50PM +0800, zijun_hu wrote: >> From: zijun_hu >> >> the LSB of a chunk->map element is used for free/in-use flag of a area >> and the other bits for offset, the sufficient and necessary condition of >> this usage is that both size and alignment of a area must be even numbers >> however, pcpu_alloc() doesn't force its @align parameter a even number >> explicitly, so a odd @align maybe causes a series of errors, see below >> example for concrete descriptions. >> >> lets assume area [16, 36) is free but its previous one is in-use, we want >> to allocate a @size == 8 and @align == 7 area. the larger area [16, 36) is >> split to three areas [16, 21), [21, 29), [29, 36) eventually. however, due >> to the usage for a chunk->map element, the actual offset of the aim area >> [21, 29) is 21 but is recorded in relevant element as 20; moreover the >> residual tail free area [29, 36) is mistook as in-use and is lost silently >> >> as explained above, inaccurate either offset or free/in-use state of >> a area is recorded into relevant chunk->map element if request a odd >> alignment area, and so causes memory leakage issue >> >> fix it by forcing the @align of a area to allocate a even number >> as do for @size. >> >> BTW, macro ALIGN() within pcpu_fit_in_area() is replaced by roundup() too >> due to back reason. in order to align a value @v up to @a boundary, macro >> roundup(v, a) is more generic than ALIGN(x, a); the latter doesn't work >> well when @a isn't a power of 2 value. for example, roundup(10, 6) == 12 >> but ALIGN(10, 6) == 10, the former result is desired obviously >> >> Signed-off-by: zijun_hu > > Nacked-by: Tejun Heo > > This is a fix for an imaginary problem. The most we should do about > odd alignment is triggering a WARN_ON. > for the current code, only power of 2 alignment value can works well is it acceptable to performing a power of 2 checking and returning error code if fail? -- 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