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 X-Spam-Level: X-Spam-Status: No, score=-7.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2AE0DC433E0 for ; Sun, 9 Aug 2020 02:31:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 023D82067D for ; Sun, 9 Aug 2020 02:31:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="RqkMrHDO" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726262AbgHICbr (ORCPT ); Sat, 8 Aug 2020 22:31:47 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:22207 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726097AbgHICbq (ORCPT ); Sat, 8 Aug 2020 22:31:46 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1596940303; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=MbLkPd5h+w3HuYbUtFGo2ztSxrORHhNasP5E8T/5k6w=; b=RqkMrHDOg/9TazNBVVmkPuJYyotu1WGscoFKAXHusiJ4qyNhW54scke0b0Ct77HS7mHM+h iR0zSrTdhB9kkr8ToERtYOrMETuo7Xm/TJ1MhXJ9kGhQeZL/yeLM/XvH8uFZsULW69l/xr HvAN+COp2JhEXvcqALhMxPtUep4kA/w= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-260-lJlH5iseOYKkf00s_8JW6w-1; Sat, 08 Aug 2020 22:31:39 -0400 X-MC-Unique: lJlH5iseOYKkf00s_8JW6w-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id EA29E79EC0; Sun, 9 Aug 2020 02:31:37 +0000 (UTC) Received: from T590 (ovpn-12-63.pek2.redhat.com [10.72.12.63]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 82CD25F9DC; Sun, 9 Aug 2020 02:31:28 +0000 (UTC) Date: Sun, 9 Aug 2020 10:31:23 +0800 From: Ming Lei To: Matthew Wilcox Cc: Al Viro , Tetsuo Handa , syzkaller-bugs@googlegroups.com, syzbot , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: splice: infinite busy loop lockup bug Message-ID: <20200809023123.GB2134904@T590> References: <00000000000084b59f05abe928ee@google.com> <29de15ff-15e9-5c52-cf87-e0ebdfa1a001@I-love.SAKURA.ne.jp> <20200807122727.GR1236603@ZenIV.linux.org.uk> <20200807123854.GS1236603@ZenIV.linux.org.uk> <20200807134114.GA2114050@T590> <20200807141148.GD17456@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200807141148.GD17456@casper.infradead.org> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 07, 2020 at 03:11:48PM +0100, Matthew Wilcox wrote: > On Fri, Aug 07, 2020 at 09:41:14PM +0800, Ming Lei wrote: > > On Fri, Aug 07, 2020 at 01:38:54PM +0100, Al Viro wrote: > > > FWIW, my preference would be to have for_each_bvec() advance past zero-length > > > segments; I'll need to go through its uses elsewhere in the tree first, though > > > (after I grab some sleep), > > > > Usually block layer doesn't allow/support zero bvec, however we can make > > for_each_bvec() to support it only. > > > > Tetsuo, can you try the following patch? > > > > diff --git a/include/linux/bvec.h b/include/linux/bvec.h > > index ac0c7299d5b8..b03c793dd28d 100644 > > --- a/include/linux/bvec.h > > +++ b/include/linux/bvec.h > > @@ -117,11 +117,19 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv, > > return true; > > } > > > > +static inline void bvec_iter_skip_zero_vec(const struct bio_vec *bv, > > + struct bvec_iter *iter) > > +{ > > + iter->bi_idx++; > > + iter->bi_bvec_done = 0; > > +} > > + > > #define for_each_bvec(bvl, bio_vec, iter, start) \ > > for (iter = (start); \ > > (iter).bi_size && \ > > - ((bvl = bvec_iter_bvec((bio_vec), (iter))), 1); \ > > - bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len)) > > + ((bvl = bvec_iter_bvec((bio_vec), (iter))), 1); \ > > + (bvl).bv_len ? bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len) : \ > > + bvec_iter_skip_zero_vec((bio_vec), &(iter))) > > Uhm, bvec_iter_advance() already skips over zero length bio_vecs. > > while (bytes && bytes >= bv[idx].bv_len) { > bytes -= bv[idx].bv_len; > idx++; > } The issue is that zero (bvl).bv_len passed to bvec_iter_advance(), so the iterator can't move on. And I tried to avoid change to bvec_iter_advance() since this exact issue only exists on for_each_bvec, and block layer won't support/allow zero-length bvec. > > The problem is when the _first_ bio_vec is zero length. It can be any zero-length bvec during the iterating. > Maybe something more > like this (which doesn't even compile, but hopefully makes my point): > > @@ -86,12 +86,24 @@ struct bvec_iter_all { > (mp_bvec_iter_page((bvec), (iter)) + \ > mp_bvec_iter_page_idx((bvec), (iter))) > > -#define bvec_iter_bvec(bvec, iter) \ > -((struct bio_vec) { \ > - .bv_page = bvec_iter_page((bvec), (iter)), \ > - .bv_len = bvec_iter_len((bvec), (iter)), \ > - .bv_offset = bvec_iter_offset((bvec), (iter)), \ > -}) > +static inline bool bvec_iter_bvec(struct bio_vec *bv, struct bio_vec *bvec, > + struct bvec_iter *iter) > +{ > + unsigned int idx = iter->bi_idx; > + > + if (!iter->bi_size) > + return false; > + > + while (!bv[idx].bv_len) > + idx++; > + iter->bi_idx = idx; > + > + bv->bv_page = bvec_iter_page(bvec, *iter); > + bv->bv_len = bvec_iter_len(bvec, *iter); > + bv->bv_offset = bvec_iter_offset(bvec, *iter); > + > + return true; > +} > > static inline bool bvec_iter_advance(const struct bio_vec *bv, > struct bvec_iter *iter, unsigned bytes) > @@ -119,8 +131,7 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv, > > #define for_each_bvec(bvl, bio_vec, iter, start) \ > for (iter = (start); \ > - (iter).bi_size && \ > - ((bvl = bvec_iter_bvec((bio_vec), (iter))), 1); \ > + bvec_iter_bvec(&(bvl), (bio_vec), &(iter)); \ > bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len)) > > /* for iterating one bio from start to end */ > > (I find the whole bvec handling a mess of confusing macros and would > welcome more of it being inline functions, in general). The above change may bring more code duplication. Meantime, it can't work because (bvl).bv_len isn't taken into account into bvec_iter_bvec(), then how can the iterator advance? Thanks, Ming