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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 61B5BC433EF for ; Mon, 21 Mar 2022 14:51:17 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DF9CB6B0072; Mon, 21 Mar 2022 10:51:16 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D828B6B0073; Mon, 21 Mar 2022 10:51:16 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BFD3F8D0001; Mon, 21 Mar 2022 10:51:16 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0187.hostedemail.com [216.40.44.187]) by kanga.kvack.org (Postfix) with ESMTP id A97916B0072 for ; Mon, 21 Mar 2022 10:51:16 -0400 (EDT) Received: from smtpin24.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 650BD181CBDBC for ; Mon, 21 Mar 2022 14:51:16 +0000 (UTC) X-FDA: 79268681352.24.DDF8744 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by imf08.hostedemail.com (Postfix) with ESMTP id DD45E160021 for ; Mon, 21 Mar 2022 14:51:15 +0000 (UTC) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 5AF441F37C; Mon, 21 Mar 2022 14:51:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1647874274; h=from:from:reply-to: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=mbrDCR0J5XThblCguHSJHDsBNerL7VvUKIeoVcpySS8=; b=NJpCAfgkBTFzQ1hNrQHXNjpeVPjy7qPKmPeVrJXXXgmjX20M1MXMsFfZm+P4Nq/C+AmlUL DDi7Abt6TBRanpE1BC3bpw811nZxUH78/cgqDdh9ja4UnRsutBsV9DMf8/jdvG7eBjayUk p5aBzGnInqzCKrcNmTdDznGKtdHrDIw= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1647874274; h=from:from:reply-to: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=mbrDCR0J5XThblCguHSJHDsBNerL7VvUKIeoVcpySS8=; b=wj2qKJ1KhORR++aHSIwKb63HUhlMn1ktDnai8hKNbRnqw7QtLZQCJIxubn1bs2zZAie/q2 H8dVuDHFpVn4NoBg== Received: from quack3.suse.cz (unknown [10.100.224.230]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 9B2F8A3B88; Mon, 21 Mar 2022 14:51:13 +0000 (UTC) Received: by quack3.suse.cz (Postfix, from userid 1000) id B2A77A0610; Mon, 21 Mar 2022 15:51:11 +0100 (CET) Date: Mon, 21 Mar 2022 15:51:11 +0100 From: Jan Kara To: Amir Goldstein Cc: Jan Kara , Trond Myklebust , "bfields@fieldses.org" , "khazhy@google.com" , "chuck.lever@oracle.com" , "linux-mm@kvack.org" , "linux-nfs@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Jeff Layton Subject: Re: [PATCH RFC] nfsd: avoid recursive locking through fsnotify Message-ID: <20220321145111.qz3bngofoi5r5cmh@quack3.lan> References: <20220319001635.4097742-1-khazhy@google.com> <20220321112310.vpr7oxro2xkz5llh@quack3.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: 4yg7smafg3khm4hywmew8jfow44s4esy Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=NJpCAfgk; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=wj2qKJ1K; spf=pass (imf08.hostedemail.com: domain of jack@suse.cz designates 195.135.220.29 as permitted sender) smtp.mailfrom=jack@suse.cz; dmarc=none X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: DD45E160021 X-HE-Tag: 1647874275-569828 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon 21-03-22 13:56:47, Amir Goldstein wrote: > On Mon, Mar 21, 2022 at 1:23 PM Jan Kara wrote: > > > > On Sat 19-03-22 11:36:13, Amir Goldstein wrote: > > > On Sat, Mar 19, 2022 at 9:02 AM Trond Myklebust wrote: > > > > > > > > On Fri, 2022-03-18 at 17:16 -0700, Khazhismel Kumykov wrote: > > > > > fsnotify_add_inode_mark may allocate with GFP_KERNEL, which may > > > > > result > > > > > in recursing back into nfsd, resulting in deadlock. See below stack. > > > > > > > > > > nfsd D 0 1591536 2 0x80004080 > > > > > Call Trace: > > > > > __schedule+0x497/0x630 > > > > > schedule+0x67/0x90 > > > > > schedule_preempt_disabled+0xe/0x10 > > > > > __mutex_lock+0x347/0x4b0 > > > > > fsnotify_destroy_mark+0x22/0xa0 > > > > > nfsd_file_free+0x79/0xd0 [nfsd] > > > > > nfsd_file_put_noref+0x7c/0x90 [nfsd] > > > > > nfsd_file_lru_dispose+0x6d/0xa0 [nfsd] > > > > > nfsd_file_lru_scan+0x57/0x80 [nfsd] > > > > > do_shrink_slab+0x1f2/0x330 > > > > > shrink_slab+0x244/0x2f0 > > > > > shrink_node+0xd7/0x490 > > > > > do_try_to_free_pages+0x12f/0x3b0 > > > > > try_to_free_pages+0x43f/0x540 > > > > > __alloc_pages_slowpath+0x6ab/0x11c0 > > > > > __alloc_pages_nodemask+0x274/0x2c0 > > > > > alloc_slab_page+0x32/0x2e0 > > > > > new_slab+0xa6/0x8b0 > > > > > ___slab_alloc+0x34b/0x520 > > > > > kmem_cache_alloc+0x1c4/0x250 > > > > > fsnotify_add_mark_locked+0x18d/0x4c0 > > > > > fsnotify_add_mark+0x48/0x70 > > > > > nfsd_file_acquire+0x570/0x6f0 [nfsd] > > > > > nfsd_read+0xa7/0x1c0 [nfsd] > > > > > nfsd3_proc_read+0xc1/0x110 [nfsd] > > > > > nfsd_dispatch+0xf7/0x240 [nfsd] > > > > > svc_process_common+0x2f4/0x610 [sunrpc] > > > > > svc_process+0xf9/0x110 [sunrpc] > > > > > nfsd+0x10e/0x180 [nfsd] > > > > > kthread+0x130/0x140 > > > > > ret_from_fork+0x35/0x40 > > > > > > > > > > Signed-off-by: Khazhismel Kumykov > > > > > --- > > > > > fs/nfsd/filecache.c | 4 ++++ > > > > > 1 file changed, 4 insertions(+) > > > > > > > > > > Marking this RFC since I haven't actually had a chance to test this, > > > > > we > > > > > we're seeing this deadlock for some customers. > > > > > > > > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c > > > > > index fdf89fcf1a0c..a14760f9b486 100644 > > > > > --- a/fs/nfsd/filecache.c > > > > > +++ b/fs/nfsd/filecache.c > > > > > @@ -121,6 +121,7 @@ nfsd_file_mark_find_or_create(struct nfsd_file > > > > > *nf) > > > > > struct fsnotify_mark *mark; > > > > > struct nfsd_file_mark *nfm = NULL, *new; > > > > > struct inode *inode = nf->nf_inode; > > > > > + unsigned int pflags; > > > > > > > > > > do { > > > > > mutex_lock(&nfsd_file_fsnotify_group->mark_mutex); > > > > > @@ -149,7 +150,10 @@ nfsd_file_mark_find_or_create(struct nfsd_file > > > > > *nf) > > > > > new->nfm_mark.mask = FS_ATTRIB|FS_DELETE_SELF; > > > > > refcount_set(&new->nfm_ref, 1); > > > > > > > > > > + /* fsnotify allocates, avoid recursion back into nfsd > > > > > */ > > > > > + pflags = memalloc_nofs_save(); > > > > > err = fsnotify_add_inode_mark(&new->nfm_mark, inode, > > > > > 0); > > > > > + memalloc_nofs_restore(pflags); > > > > > > > > > > /* > > > > > * If the add was successful, then return the object. > > > > > > > > Isn't that stack trace showing a slab direct reclaim, and not a > > > > filesystem writeback situation? > > > > > > > > Does memalloc_nofs_save()/restore() really fix this problem? It seems > > > > to me that it cannot, particularly since knfsd is not a filesystem, and > > > > so does not ever handle writeback of dirty pages. > > > > > > > > > > Maybe NOFS throttles direct reclaims to the point that the problem is > > > harder to hit? > > > > > > This report came in at good timing for me. > > > > > > It demonstrates an issue I did not predict for "volatile"' fanotify marks [1]. > > > As far as I can tell, nfsd filecache is currently the only fsnotify backend that > > > frees fsnotify marks in memory shrinker. "volatile" fanotify marks would also > > > be evictable in that way, so they would expose fanotify to this deadlock. > > > > > > For the short term, maybe nfsd filecache can avoid the problem by checking > > > mutex_is_locked(&nfsd_file_fsnotify_group->mark_mutex) and abort the > > > shrinker. I wonder if there is a place for a helper mutex_is_locked_by_me()? > > > > > > Jan, > > > > > > A relatively simple fix would be to allocate fsnotify_mark_connector in > > > fsnotify_add_mark() and free it, if a connector already exists for the object. > > > I don't think there is a good reason to optimize away this allocation > > > for the case of a non-first group to set a mark on an object? > > > > Indeed, nasty. Volatile marks will add group->mark_mutex into a set of > > locks grabbed during inode slab reclaim. So any allocation under > > group->mark_mutex has to be GFP_NOFS now. This is not just about connector > > allocations but also mark allocations for fanotify. Moving allocations from > > under mark_mutex is also possible solution but passing preallocated memory > > around is kind of ugly as well. > > Yes, kind of, here is how it looks: > https://github.com/amir73il/linux/commit/643bb6b9f664f70f68ea0393a06338673c4966b3 > https://github.com/amir73il/linux/commit/66f27fc99e46b12f1078e8e2915793040ce50ee7 Yup, not an act of beauty but bearable in the worst case :). > > So the cleanest solution I currently see is > > to come up with helpers like "fsnotify_lock_group() & > > fsnotify_unlock_group()" which will lock/unlock mark_mutex and also do > > memalloc_nofs_save / restore magic. > > > > Sounds good. Won't this cause a regression - more failures to setup new mark > under memory pressure? Well, yes, the chances of hitting ENOMEM under heavy memory pressure are higher. But I don't think that much memory is consumed by connectors or marks that the reduced chances for direct reclaim would really substantially matter for the system as a whole. > Should we maintain a flag in the group FSNOTIFY_GROUP_SHRINKABLE? > and set NOFS state only in that case, so at least we don't cause regression > for existing applications? So that's a possibility I've left in my sleeve ;). We could do it but then we'd also have to tell lockdep that there are two kinds of mark_mutex locks so that it does not complain about possible reclaim deadlocks. Doable but at this point I didn't consider it worth it unless someone comes with a bug report from a real user scenario. Honza -- Jan Kara SUSE Labs, CR