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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 86097C5B57B for ; Sat, 29 Jun 2019 22:34:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5D828215EA for ; Sat, 29 Jun 2019 22:34:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726952AbfF2WeY (ORCPT ); Sat, 29 Jun 2019 18:34:24 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:33232 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726925AbfF2WeX (ORCPT ); Sat, 29 Jun 2019 18:34:23 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92 #3 (Red Hat Linux)) id 1hhLv0-00037Q-Th; Sat, 29 Jun 2019 22:34:02 +0000 Date: Sat, 29 Jun 2019 23:34:02 +0100 From: Al Viro To: Linus Torvalds Cc: "Tobin C. Harding" , Andrew Morton , Roman Gushchin , Alexander Viro , Christoph Hellwig , Pekka Enberg , David Rientjes , Joonsoo Kim , Christopher Lameter , Matthew Wilcox , Miklos Szeredi , Andreas Dilger , Waiman Long , Tycho Andersen , Theodore Ts'o , Andi Kleen , David Chinner , Nick Piggin , Rik van Riel , Hugh Dickins , Jonathan Corbet , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: shrink_dentry_list() logics change (was Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects) Message-ID: <20190629223402.GX17978@ZenIV.linux.org.uk> References: <20190411013441.5415-1-tobin@kernel.org> <20190411013441.5415-15-tobin@kernel.org> <20190411023322.GD2217@ZenIV.linux.org.uk> <20190411024821.GB6941@eros.localdomain> <20190411044746.GE2217@ZenIV.linux.org.uk> <20190411210200.GH2217@ZenIV.linux.org.uk> <20190629040844.GS17978@ZenIV.linux.org.uk> <20190629043803.GT17978@ZenIV.linux.org.uk> <20190629190624.GU17978@ZenIV.linux.org.uk> <20190629222945.GW17978@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190629222945.GW17978@ZenIV.linux.org.uk> User-Agent: Mutt/1.11.3 (2019-02-01) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Sat, Jun 29, 2019 at 11:29:45PM +0100, Al Viro wrote: > Like this (again, only build-tested): ... and with obvious braino fixed, Teach shrink_dcache_parent() to cope with mixed-filesystem shrink lists Currently, running into a shrink list that contains dentries from different filesystems can cause several unpleasant things for shrink_dcache_parent() and for umount(2). The first problem is that there's a window during shrink_dentry_list() between __dentry_kill() takes a victim out and dropping reference to its parent. During that window the parent looks like a genuine busy dentry. shrink_dcache_parent() (or, worse yet, shrink_dcache_for_umount()) coming at that time will see no eviction candidates and no indication that it needs to wait for some shrink_dentry_list() to proceed further. That applies for any shrink list that might intersect with the subtree we are trying to shrink; the only reason it does not blow on umount(2) in the mainline is that we unregister the memory shrinker before hitting shrink_dcache_for_umount(). Another problem happens if something in a mixed-filesystem shrink list gets be stuck in e.g. iput(), getting umount of unrelated fs to spin waiting for the stuck shrinker to get around to our dentries. Solution: 1) have shrink_dentry_list() decrement the parent's refcount and make sure it's on a shrink list (ours unless it already had been on some other) before calling __dentry_kill(). That eliminates the window when shrink_dcache_parent() would've blown past the entire subtree without noticing anything with zero refcount not on shrink lists. 2) when shrink_dcache_parent() has found no eviction candidates, but some dentries are still sitting on shrink lists, rather than repeating the scan in hope that shrinkers have progressed, scan looking for something on shrink lists with zero refcount. If such a thing is found, grab rcu_read_lock() and stop the scan, with caller locking it for eviction, dropping out of RCU and doing __dentry_kill(), with the same treatment for parent as shrink_dentry_list() would do. Note that right now mixed-filesystem shrink lists do not occur, so this is not a mainline bug. Howevere, there's a bunch of uses for such beasts (e.g. the "try and evict everything we can out of given page" patches; there are potential uses in mount-related code, considerably simplifying the life in fs/namespace.c, etc.) Signed-off-by: Al Viro --- diff --git a/fs/dcache.c b/fs/dcache.c index 8136bda27a1f..4b50e09ee950 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -860,6 +860,32 @@ void dput(struct dentry *dentry) } EXPORT_SYMBOL(dput); +static void __dput_to_list(struct dentry *dentry, struct list_head *list) +__must_hold(&dentry->d_lock) +{ + if (dentry->d_flags & DCACHE_SHRINK_LIST) { + /* let the owner of the list it's on deal with it */ + --dentry->d_lockref.count; + } else { + if (dentry->d_flags & DCACHE_LRU_LIST) + d_lru_del(dentry); + if (!--dentry->d_lockref.count) + d_shrink_add(dentry, list); + } +} + +void dput_to_list(struct dentry *dentry, struct list_head *list) +{ + rcu_read_lock(); + if (likely(fast_dput(dentry))) { + rcu_read_unlock(); + return; + } + rcu_read_unlock(); + if (!retain_dentry(dentry)) + __dput_to_list(dentry, list); + spin_unlock(&dentry->d_lock); +} /* This must be called with d_lock held */ static inline void __dget_dlock(struct dentry *dentry) @@ -1088,18 +1114,9 @@ static void shrink_dentry_list(struct list_head *list) rcu_read_unlock(); d_shrink_del(dentry); parent = dentry->d_parent; + if (parent != dentry) + __dput_to_list(parent, list); __dentry_kill(dentry); - if (parent == dentry) - continue; - /* - * We need to prune ancestors too. This is necessary to prevent - * quadratic behavior of shrink_dcache_parent(), but is also - * expected to be beneficial in reducing dentry cache - * fragmentation. - */ - dentry = parent; - while (dentry && !lockref_put_or_lock(&dentry->d_lockref)) - dentry = dentry_kill(dentry); } } @@ -1444,8 +1461,11 @@ int d_set_mounted(struct dentry *dentry) struct select_data { struct dentry *start; + union { + long found; + struct dentry *victim; + }; struct list_head dispose; - int found; }; static enum d_walk_ret select_collect(void *_data, struct dentry *dentry) @@ -1477,6 +1497,37 @@ static enum d_walk_ret select_collect(void *_data, struct dentry *dentry) return ret; } +static enum d_walk_ret select_collect2(void *_data, struct dentry *dentry) +{ + struct select_data *data = _data; + enum d_walk_ret ret = D_WALK_CONTINUE; + + if (data->start == dentry) + goto out; + + if (dentry->d_flags & DCACHE_SHRINK_LIST) { + if (!dentry->d_lockref.count) { + rcu_read_lock(); + data->victim = dentry; + return D_WALK_QUIT; + } + } else { + if (dentry->d_flags & DCACHE_LRU_LIST) + d_lru_del(dentry); + if (!dentry->d_lockref.count) + d_shrink_add(dentry, &data->dispose); + } + /* + * We can return to the caller if we have found some (this + * ensures forward progress). We'll be coming back to find + * the rest. + */ + if (!list_empty(&data->dispose)) + ret = need_resched() ? D_WALK_QUIT : D_WALK_NORETRY; +out: + return ret; +} + /** * shrink_dcache_parent - prune dcache * @parent: parent of entries to prune @@ -1486,12 +1537,9 @@ static enum d_walk_ret select_collect(void *_data, struct dentry *dentry) void shrink_dcache_parent(struct dentry *parent) { for (;;) { - struct select_data data; + struct select_data data = {.start = parent}; INIT_LIST_HEAD(&data.dispose); - data.start = parent; - data.found = 0; - d_walk(parent, &data, select_collect); if (!list_empty(&data.dispose)) { @@ -1502,6 +1550,22 @@ void shrink_dcache_parent(struct dentry *parent) cond_resched(); if (!data.found) break; + data.victim = NULL; + d_walk(parent, &data, select_collect2); + if (data.victim) { + struct dentry *parent; + if (!shrink_lock_dentry(data.victim)) { + rcu_read_unlock(); + } else { + rcu_read_unlock(); + parent = data.victim->d_parent; + if (parent != data.victim) + __dput_to_list(parent, &data.dispose); + __dentry_kill(data.victim); + } + } + if (!list_empty(&data.dispose)) + shrink_dentry_list(&data.dispose); } } EXPORT_SYMBOL(shrink_dcache_parent); diff --git a/fs/internal.h b/fs/internal.h index 0010889f2e85..68f132cf2664 100644 --- a/fs/internal.h +++ b/fs/internal.h @@ -160,6 +160,7 @@ extern int d_set_mounted(struct dentry *dentry); extern long prune_dcache_sb(struct super_block *sb, struct shrink_control *sc); extern struct dentry *d_alloc_cursor(struct dentry *); extern struct dentry * d_alloc_pseudo(struct super_block *, const struct qstr *); +extern void dput_to_list(struct dentry *, struct list_head *); /* * read_write.c