From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f170.google.com (mail-ig0-f170.google.com [209.85.213.170]) by kanga.kvack.org (Postfix) with ESMTP id C1C166B0005 for ; Mon, 18 Jan 2016 01:30:27 -0500 (EST) Received: by mail-ig0-f170.google.com with SMTP id ik10so49497509igb.1 for ; Sun, 17 Jan 2016 22:30:27 -0800 (PST) Received: from lgeamrelo13.lge.com (LGEAMRELO13.lge.com. [156.147.23.53]) by mx.google.com with ESMTPS id vt4si24291956igb.57.2016.01.17.22.30.25 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Sun, 17 Jan 2016 22:30:27 -0800 (PST) Date: Mon, 18 Jan 2016 15:32:46 +0900 From: Minchan Kim Subject: Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition Message-ID: <20160118063246.GB7453@bbox> References: <1452843551-4464-1-git-send-email-junil0814.lee@lge.com> <20160115143434.GA25332@blaptop.local> <56991514.9000609@suse.cz> <20160116040913.GA566@swordfish> <5699F4C9.1070902@suse.cz> <20160116080650.GB566@swordfish> <5699FC69.4010000@suse.cz> MIME-Version: 1.0 In-Reply-To: <5699FC69.4010000@suse.cz> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Sergey Senozhatsky , Junil Lee , ngupta@vflare.org, sergey.senozhatsky.work@gmail.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Hello, Vlastimil On Sat, Jan 16, 2016 at 09:16:41AM +0100, Vlastimil Babka wrote: > On 16.1.2016 9:06, Sergey Senozhatsky wrote: > > On (01/16/16 08:44), Vlastimil Babka wrote: > >> On 16.1.2016 5:09, Sergey Senozhatsky wrote: > >>> On (01/15/16 16:49), Vlastimil Babka wrote: > >> > >> Hmm but that's an unpin, not a pin? A mistake or I'm missing something? > > > > I'm sure it's just a compose-in-mail-app typo. > > BTW, couldn't the correct fix also just look like this? > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > index 9f15bdd9163c..43f743175ede 100644 > --- a/mm/zsmalloc.c > +++ b/mm/zsmalloc.c > @@ -1635,8 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct > size_class *class, > free_obj = obj_malloc(d_page, class, handle); > zs_object_copy(free_obj, used_obj, class); > index++; > + /* This also effectively unpins the handle */ > record_obj(handle, free_obj); > - unpin_tag(handle); > obj_free(pool, class, used_obj); > } > > But I'd still recommend WRITE_ONCE in record_obj(). And I'm not even sure it's Thanks for the reivew. Yeah, we need WRITE_ONCE in record_obj but your version will not work. IMHO, WRITE_ONCE can prevent store-tearing but it couldn't prevent reordering. IOW, we need some barrier as unlock and clear_bit_unlock includes it. So, we shouldn't omit unpin_tag there. > safe on all architectures to do a simple overwrite of a word against somebody > else trying to lock a bit there? Hmm, I think it shouldn't be a problem. It's word-alinged, word-sized store so it should be atomic. As other example, we have been used lock_page for a bit of page->flags and used other bits in there with __set_bit(ie, __SetPageXXX). I guess it's same situation with us just except we are spinning there. But it is worth to dobule check so need to help lock guys. > > > -ss > > > >> Anyway the compiler can do the same thing here without a WRITE_ONCE(). > > > > -- > > 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 > > > -- 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