From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qc0-f169.google.com (mail-qc0-f169.google.com [209.85.216.169]) by kanga.kvack.org (Postfix) with ESMTP id ECA0B6B0031 for ; Wed, 20 Nov 2013 21:52:56 -0500 (EST) Received: by mail-qc0-f169.google.com with SMTP id u18so6523264qcx.28 for ; Wed, 20 Nov 2013 18:52:56 -0800 (PST) Received: from mail-qe0-x22f.google.com (mail-qe0-x22f.google.com [2607:f8b0:400d:c02::22f]) by mx.google.com with ESMTPS id t9si989890qat.53.2013.11.20.18.52.55 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 20 Nov 2013 18:52:56 -0800 (PST) Received: by mail-qe0-f47.google.com with SMTP id t7so2140297qeb.20 for ; Wed, 20 Nov 2013 18:52:55 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <1384976824-32624-1-git-send-email-ddstreet@ieee.org> References: <1384976824-32624-1-git-send-email-ddstreet@ieee.org> Date: Thu, 21 Nov 2013 10:52:55 +0800 Message-ID: Subject: Re: [PATCH] mm/zswap: remove unneeded zswap_rb_erase calls From: Weijie Yang Content-Type: text/plain; charset=ISO-8859-1 Sender: owner-linux-mm@kvack.org List-ID: To: Dan Streetman Cc: Seth Jennings , linux-mm@kvack.org, linux-kernel , Bob Liu , Minchan Kim , Weijie Yang Hello Dan On Thu, Nov 21, 2013 at 3:47 AM, Dan Streetman wrote: > Since zswap_rb_erase was added to the final (when refcount == 0) > zswap_put_entry, there is no need to call zswap_rb_erase before > calling zswap_put_entry. > > Signed-off-by: Dan Streetman > --- > mm/zswap.c | 5 ----- > 1 file changed, 5 deletions(-) > > diff --git a/mm/zswap.c b/mm/zswap.c > index e154f1e..f4fbbd5 100644 > --- a/mm/zswap.c > +++ b/mm/zswap.c > @@ -711,8 +711,6 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset, > ret = zswap_rb_insert(&tree->rbroot, entry, &dupentry); > if (ret == -EEXIST) { > zswap_duplicate_entry++; > - /* remove from rbtree */ > - zswap_rb_erase(&tree->rbroot, dupentry); > zswap_entry_put(tree, dupentry); > } > } while (ret == -EEXIST); If remove zswap_rb_erase, it would loop until free this dupentry. This would cause 2 proplems: 1. zswap_duplicate_entry counter is not correct 2. trigger BUG_ON in zswap_entry_put when this dupentry is being writeback, because zswap_writeback_entry will call zswap_entry_put either. So, I don't think it is a good idea to remove zswap_rb_erase call. > @@ -787,9 +785,6 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset) > return; > } > > - /* remove from rbtree */ > - zswap_rb_erase(&tree->rbroot, entry); > - > /* drop the initial reference from entry creation */ > zswap_entry_put(tree, entry); I think it is better not to remove the zswap_rb_erase call. >>From frontswap interface view, if invalidate is called, the page(and entry) should never visible to upper. If remove the zswap_rb_erase call, it is not fit this semantic. Consider the following scenario: 1. thread 0: entry A is being writeback 2. thread 1: invalidate entry A, as refcount != 0, it will still exist on rbtree. 3. thread 1: reuse entry A 's swp_entry_t, do a frontswap_store it will conflict with the entry A on the rbtree, it is not a normal duplicate store. If we place the zswap_rb_erase call in zswap_frontswap_invalidate_page, we can avoid the above scenario. So, I don't think it is a good idea to remove zswap_rb_erase call. Regards, > -- > 1.8.3.1 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- 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