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 3A108C433F5 for ; Wed, 16 Feb 2022 03:50:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238409AbiBPDuJ (ORCPT ); Tue, 15 Feb 2022 22:50:09 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:39748 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229452AbiBPDuJ (ORCPT ); Tue, 15 Feb 2022 22:50:09 -0500 Received: from zeniv-ca.linux.org.uk (zeniv-ca.linux.org.uk [IPv6:2607:5300:60:148a::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 58B4B1FA6F; Tue, 15 Feb 2022 19:49:58 -0800 (PST) Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1nKBKG-002AeZ-O2; Wed, 16 Feb 2022 03:49:56 +0000 Date: Wed, 16 Feb 2022 03:49:56 +0000 From: Al Viro To: Stephen Brennan Cc: linux-kernel@vger.kernel.org, Luis Chamberlain , Andrew Morton , Jan Kara , linux-fsdevel@vger.kernel.org, Arnd Bergmann , Amir Goldstein Subject: Re: [PATCH v2 1/4] dcache: sweep cached negative dentries to the end of list of siblings Message-ID: References: <20220209231406.187668-1-stephen.s.brennan@oracle.com> <20220209231406.187668-2-stephen.s.brennan@oracle.com> <875ypf8s5m.fsf@stepbren-lnx.us.oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, Feb 16, 2022 at 03:27:39AM +0000, Al Viro wrote: > On Tue, Feb 15, 2022 at 06:24:53PM -0800, Stephen Brennan wrote: > > > It seems to me that, if we had taken a reference on child by > > incrementing the reference count prior to unlocking it, then > > dentry_unlist could never have been called, since we would never have > > made it into __dentry_kill. child would still be on the list, and any > > cursor (or sweep_negative) list updates would now be reflected in > > child->d_child.next. But dput is definitely not safe while holding a > > lock on a parent dentry (even more so now thanks to my patch), so that > > is out of the question. > > > > Would dput_to_list be an appropriate solution to that issue? We can > > maintain a dispose list in d_walk and then for any dput which really > > drops the refcount to 0, we can handle them after d_walk is done. It > > shouldn't be that many dentries anyway. > > Interesting idea, but... what happens to behaviour of e.g. > shrink_dcache_parent()? You'd obviously need to modify the test in > select_collect(), but then the selected dentries become likely candidates > for d_walk() itself wanting to move them over to its internal shrink list. > OTOH, __dput_to_list() will just decrement the count and skip the sucker > if it's already on a shrink list... > > It might work, but it really needs a careful analysis wrt. > parallel d_walk(). What happens when you have two threads hitting > shrink_dcache_parent() on two different places, one being an ancestor > of another? That can happen in parallel, and currently it does work > correctly, but that's fairly delicate and there are places where a minor > change could turn O(n) into O(n^2), etc. > > Let me think about that - I'm not saying it's hopeless, and it > would be nice to avoid that subtlety in dentry_unlist(), but there > might be dragons. PS: another obvious change is that d_walk() would become blocking. So e.g. int path_has_submounts(const struct path *parent) { struct check_mount data = { .mnt = parent->mnt, .mounted = 0 }; read_seqlock_excl(&mount_lock); d_walk(parent->dentry, &data, path_check_mount); read_sequnlock_excl(&mount_lock); return data.mounted; } would need a rework - d_walk() is under a spinlock here. Another potential headache in that respect is d_genocide() - currently non-blocking, with this change extremely likely to do evictions. That, however, is not a problem for current in-tree callers - they are all shortly followed by shrink_dcache_parent() or equivalents. path_has_submounts(), though... I'd really hate to reintroduce the "call this on entry/call this on exit" callbacks. Perhaps it would be better to pass the dispose list to d_walk() and have the callers deal with evictions? For that matter, shrink_dcache_parent() and friends would be just fine passing the same list they are collecting into. *growl* autofs_d_automount() has it called under sbi->fs_lock. So we'd need to take the disposal all the way out there, and export shrink_dentry_list() while we are at it. Not pretty ;-/ And no, we can't make the disposal async, so offloading it to a worker or thread is not feasible...