From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B8FA5C433EF for ; Fri, 28 Jan 2022 22:28:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241343AbiA1W2P (ORCPT ); Fri, 28 Jan 2022 17:28:15 -0500 Received: from sandeen.net ([63.231.237.45]:43020 "EHLO sandeen.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235188AbiA1W1g (ORCPT ); Fri, 28 Jan 2022 17:27:36 -0500 Received: from [10.0.0.147] (liberator.sandeen.net [10.0.0.147]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by sandeen.net (Postfix) with ESMTPSA id B5E2D78FD; Fri, 28 Jan 2022 16:27:22 -0600 (CST) Message-ID: <3c99180d-4ae2-a2dc-003d-c8efbdc1fd7f@sandeen.net> Date: Fri, 28 Jan 2022 16:27:35 -0600 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.5.1 Subject: Re: [PATCH 41/45] libxfs: always initialize internal buffer map Content-Language: en-US To: "Darrick J. Wong" Cc: linux-xfs@vger.kernel.org References: <164263784199.860211.7509808171577819673.stgit@magnolia> <164263806915.860211.11553766371419430734.stgit@magnolia> <20220128220325.GH13540@magnolia> From: Eric Sandeen In-Reply-To: <20220128220325.GH13540@magnolia> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On 1/28/22 4:03 PM, Darrick J. Wong wrote: > On Fri, Jan 28, 2022 at 02:31:11PM -0600, Eric Sandeen wrote: >> On 1/19/22 6:21 PM, Darrick J. Wong wrote: >>> From: Darrick J. Wong >>> >>> The __initbuf function is responsible for initializing the fields of an >>> xfs_buf. Buffers are always required to have a mapping, though in the >>> typical case there's only one mapping, so we can use the internal one. >>> >>> The single-mapping b_maps init code at the end of the function doesn't >>> quite get this right though -- if a single-mapping buffer in the cache >>> was allowed to expire and now is being repurposed, it'll come out with >>> b_maps == &__b_map, in which case we incorrectly skip initializing the >>> map. >> >> In this case b_nmaps must already be 1, right. And it's the bn and >> length in b_maps[0] that fail to be initialized? >> >> I wonder, then, if it's any more clear to reorganize it just a little bit, >> like: >> >> if (!bp->b_maps) { >> bp->b_maps = &bp->__b_map; >> bp->b_nmaps = 1; >> } >> >> if (bp->b_maps == &bp->__b_map) { >> bp->b_maps[0].bm_bn = bp->b_bn; >> bp->b_maps[0].bm_len = bp->b_length; >> } >> >> because AFAICT b_nmaps only needs to be reset to 1 if we didn't already >> get here with b_maps == &__b_map? > > That would also work, though it's less obvious (to me anyway) that > b_nmaps is always 1 when bp->b_maps == &bp->__b_map. Ok. I'll leave it alone. -Eric > > --D > >> If this is just navel-gazing I can leave it as is. If you think it's >> any clearer, I'll make the change. (or if I've gotten it completely wrong, >> sorry!) >> >> Thanks, >> -Eric >> >>> This has gone unnoticed until now because (AFAICT) the code paths >>> that use b_maps are the same ones that are called with multi-mapping >>> buffers, which are initialized correctly. >>> >>> Anyway, the improperly initialized single-mappings will cause problems >>> in upcoming patches where we turn b_bn into the cache key and require >>> the use of b_maps[0].bm_bn for the buffer LBA. Fix this. >>> >>> Signed-off-by: Darrick J. Wong >>> --- >>> libxfs/rdwr.c | 6 ++++-- >>> 1 file changed, 4 insertions(+), 2 deletions(-) >>> >>> >>> diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c >>> index 5086bdbc..a55e3a79 100644 >>> --- a/libxfs/rdwr.c >>> +++ b/libxfs/rdwr.c >>> @@ -251,9 +251,11 @@ __initbuf(struct xfs_buf *bp, struct xfs_buftarg *btp, xfs_daddr_t bno, >>> bp->b_ops = NULL; >>> INIT_LIST_HEAD(&bp->b_li_list); >>> - if (!bp->b_maps) { >>> - bp->b_nmaps = 1; >>> + if (!bp->b_maps) >>> bp->b_maps = &bp->__b_map; >>> + >>> + if (bp->b_maps == &bp->__b_map) { >>> + bp->b_nmaps = 1; >>> bp->b_maps[0].bm_bn = bp->b_bn; >>> bp->b_maps[0].bm_len = bp->b_length; >>> } >>> >