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=-8.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 CB8EEC433E0 for ; Mon, 25 Jan 2021 18:31:05 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 02D9922B3F for ; Mon, 25 Jan 2021 18:31:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 02D9922B3F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 4EB3D6B0078; Mon, 25 Jan 2021 13:31:03 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 49A9E6B007B; Mon, 25 Jan 2021 13:31:03 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 362BB6B007D; Mon, 25 Jan 2021 13:31:03 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0228.hostedemail.com [216.40.44.228]) by kanga.kvack.org (Postfix) with ESMTP id 216D06B0078 for ; Mon, 25 Jan 2021 13:31:03 -0500 (EST) Received: from smtpin26.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id CC50E3628 for ; Mon, 25 Jan 2021 18:31:02 +0000 (UTC) X-FDA: 77745139164.26.play77_580814927587 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin26.hostedemail.com (Postfix) with ESMTP id ADC1F1804B669 for ; Mon, 25 Jan 2021 18:31:02 +0000 (UTC) X-HE-Tag: play77_580814927587 X-Filterd-Recvd-Size: 9980 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf07.hostedemail.com (Postfix) with ESMTP for ; Mon, 25 Jan 2021 18:31:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=T1UWtlNZr7Dokdzr3mVGS/edHEQG76PknlPWffcpm4k=; b=ORyY0SwLbNumNsaGfMQOC1fNeF opNajgOJtJ0+wMvbeVy/V3NjEu9tUUUEEk1qW/KcU/0/Pb9i8NJFiqpqn599ch+76fi2ua4uji3AH Ze6uAWxivrLFUbg3Casz4FKJe0ZUFdeAUOKRPKqrcxWe3rfekFa93w/1DXzKWdoKLFbidQwE49mPq gX31XgRfdapRiEMeuujfVOxBCWMEcbC+xMi5cak4b03s8nUS7F1aqj4lhkEmbCWbdCYXa+DyBtgvV lxnCRviQVch2r5o62HYxuHG7ZUyYnJIMuFqFz0lkrUVV9gbYevICMNMKYcupjD+Fif5daAjRfZ7sp I8SBhupg==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1l46c5-004WEQ-PX; Mon, 25 Jan 2021 18:29:31 +0000 Date: Mon, 25 Jan 2021 18:29:21 +0000 From: Matthew Wilcox To: Waiman Long Cc: Michal Hocko , Andrew Morton , Johannes Weiner , Alex Shi , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/filemap: Adding missing mem_cgroup_uncharge() to __add_to_page_cache_locked() Message-ID: <20210125182921.GG308988@casper.infradead.org> References: <20210125042441.20030-1-longman@redhat.com> <20210125092815.GB827@dhcp22.suse.cz> <20210125160328.GP827@dhcp22.suse.cz> <20210125162506.GF308988@casper.infradead.org> <20210125164118.GS827@dhcp22.suse.cz> <20210125181436.GV827@dhcp22.suse.cz> <53eb7692-e559-a914-e103-adfe951d7a7c@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <53eb7692-e559-a914-e103-adfe951d7a7c@redhat.com> Content-Transfer-Encoding: quoted-printable 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, Jan 25, 2021 at 01:23:58PM -0500, Waiman Long wrote: > On 1/25/21 1:14 PM, Michal Hocko wrote: > > On Mon 25-01-21 17:41:19, Michal Hocko wrote: > > > On Mon 25-01-21 16:25:06, Matthew Wilcox wrote: > > > > On Mon, Jan 25, 2021 at 05:03:28PM +0100, Michal Hocko wrote: > > > > > On Mon 25-01-21 10:57:54, Waiman Long wrote: > > > > > > On 1/25/21 4:28 AM, Michal Hocko wrote: > > > > > > > On Sun 24-01-21 23:24:41, Waiman Long wrote: > > > > > > > > The commit 3fea5a499d57 ("mm: memcontrol: convert page > > > > > > > > cache to a new mem_cgroup_charge() API") introduced a bug= in > > > > > > > > __add_to_page_cache_locked() causing the following splat: > > > > > > > >=20 > > > > > > > > [ 1570.068330] page dumped because: VM_BUG_ON_PAGE(pag= e_memcg(page)) > > > > > > > > [ 1570.068333] pages's memcg:ffff8889a4116000 > > > > > > > > [ 1570.068343] ------------[ cut here ]------------ > > > > > > > > [ 1570.068346] kernel BUG at mm/memcontrol.c:2924! > > > > > > > > [ 1570.068355] invalid opcode: 0000 [#1] SMP KASAN PTI > > > > > > > > [ 1570.068359] CPU: 35 PID: 12345 Comm: cat Tainted: G= S W I 5.11.0-rc4-debug+ #1 > > > > > > > > [ 1570.068363] Hardware name: HP HP Z8 G4 Workstation/= 81C7, BIOS P60 v01.25 12/06/2017 > > > > > > > > [ 1570.068365] RIP: 0010:commit_charge+0xf4/0x130 > > > > > > > > : > > > > > > > > [ 1570.068375] RSP: 0018:ffff8881b38d70e8 EFLAGS: 0001= 0286 > > > > > > > > [ 1570.068379] RAX: 0000000000000000 RBX: ffffea00260d= dd00 RCX: 0000000000000027 > > > > > > > > [ 1570.068382] RDX: 0000000000000000 RSI: 000000000000= 0004 RDI: ffff88907ebe05a8 > > > > > > > > [ 1570.068384] RBP: ffffea00260ddd00 R08: ffffed120fd7= c0b6 R09: ffffed120fd7c0b6 > > > > > > > > [ 1570.068386] R10: ffff88907ebe05ab R11: ffffed120fd7= c0b5 R12: ffffea00260ddd38 > > > > > > > > [ 1570.068389] R13: ffff8889a4116000 R14: ffff8889a411= 6000 R15: 0000000000000001 > > > > > > > > [ 1570.068391] FS: 00007ff039638680(0000) GS:ffff8890= 7ea00000(0000) knlGS:0000000000000000 > > > > > > > > [ 1570.068394] CS: 0010 DS: 0000 ES: 0000 CR0: 000000= 0080050033 > > > > > > > > [ 1570.068396] CR2: 00007f36f354cc20 CR3: 00000008a012= 6006 CR4: 00000000007706e0 > > > > > > > > [ 1570.068398] DR0: 0000000000000000 DR1: 000000000000= 0000 DR2: 0000000000000000 > > > > > > > > [ 1570.068400] DR3: 0000000000000000 DR6: 00000000fffe= 0ff0 DR7: 0000000000000400 > > > > > > > > [ 1570.068402] PKRU: 55555554 > > > > > > > > [ 1570.068404] Call Trace: > > > > > > > > [ 1570.068407] mem_cgroup_charge+0x175/0x770 > > > > > > > > [ 1570.068413] __add_to_page_cache_locked+0x712/0xad0 > > > > > > > > [ 1570.068439] add_to_page_cache_lru+0xc5/0x1f0 > > > > > > > > [ 1570.068461] cachefiles_read_or_alloc_pages+0x895/0= x2e10 [cachefiles] > > > > > > > > [ 1570.068524] __fscache_read_or_alloc_pages+0x6c0/0x= a00 [fscache] > > > > > > > > [ 1570.068540] __nfs_readpages_from_fscache+0x16d/0x6= 30 [nfs] > > > > > > > > [ 1570.068585] nfs_readpages+0x24e/0x540 [nfs] > > > > > > > > [ 1570.068693] read_pages+0x5b1/0xc40 > > > > > > > > [ 1570.068711] page_cache_ra_unbounded+0x460/0x750 > > > > > > > > [ 1570.068729] generic_file_buffered_read_get_pages+0= x290/0x1710 > > > > > > > > [ 1570.068756] generic_file_buffered_read+0x2a9/0xc30 > > > > > > > > [ 1570.068832] nfs_file_read+0x13f/0x230 [nfs] > > > > > > > > [ 1570.068872] new_sync_read+0x3af/0x610 > > > > > > > > [ 1570.068901] vfs_read+0x339/0x4b0 > > > > > > > > [ 1570.068909] ksys_read+0xf1/0x1c0 > > > > > > > > [ 1570.068920] do_syscall_64+0x33/0x40 > > > > > > > > [ 1570.068926] entry_SYSCALL_64_after_hwframe+0x44/0x= a9 > > > > > > > > [ 1570.068930] RIP: 0033:0x7ff039135595 > > > > > > > >=20 > > > > > > > > Before that commit, there was a try_charge() and commit_c= harge() > > > > > > > > in __add_to_page_cache_locked(). These 2 separated charge= functions > > > > > > > > were replaced by a single mem_cgroup_charge(). However, i= t forgot > > > > > > > > to add a matching mem_cgroup_uncharge() when the xarray i= nsertion > > > > > > > > failed with the page released back to the pool. Fix this = by adding a > > > > > > > > mem_cgroup_uncharge() call when insertion error happens. > > > > > > > >=20 > > > > > > > > Fixes: 3fea5a499d57 ("mm: memcontrol: convert page cache = to a new mem_cgroup_charge() API") > > > > > > > > Signed-off-by: Waiman Long > > > > > > > OK, this is indeed a subtle bug. The patch aimed at simplif= ying the > > > > > > > charge lifetime so that users do not really have to think a= bout when to > > > > > > > uncharge as that happens when the page is freed. fscache so= mehow breaks > > > > > > > that assumption because it doesn't free up pages but it kee= ps some of > > > > > > > them in the cache. > > > > > > >=20 > > > > > > > I have tried to wrap my head around the cached object life = time in > > > > > > > fscache but failed and got lost in the maze. Is this the on= ly instance > > > > > > > of the problem? Would it make more sense to explicitly hand= le charges in > > > > > > > the fscache code or there are other potential users to fall= into this > > > > > > > trap? > > > > > > There may be other places that have similar problem. I focus = on the > > > > > > filemap.c case as I have a test case that can reliably produc= e the bug > > > > > > splat. This patch does fix it for my test case. > > > > > I believe this needs a more general fix than catching a random = places > > > > > which you can trigger. Would it make more sense to address this= at the > > > > > fscache level and always make sure that a page returned to the = pool is > > > > > always uncharged instead? > > > > I believe you mean "page cache" -- there is a separate thing call= ed > > > > 'fscache' which is used to cache network filesystems. > > > Yes, I really had fscache in mind because it does have an "unusual"= page > > > life time rules. > > >=20 > > > > I don't understand the memcg code at all, so I have no useful fee= dback > > > > on what you're saying other than this. > > > Well the memcg accounting rules after the rework should have simpli= fied > > > the API usage for most users. You will get memory charged when it i= s > > > used and it will go away when the page is freed. If a page is not r= eally > > > freed in some cases and it can be reused then it doesn't really fit= into > > > this scheme automagically. I do undestand that this puts some addit= ional > > > burden on those special cases. I am not really sure what is the rig= ht > > > way here myself but considering there might be other similar cases = like > > > that I would lean towards special casing where the pool is implemen= ted. > > > I would expect there is some state to be maintain for that purpose > > > already. > > After some more thinking I've came to conclusion that the patch as > > proposed is the proper way forward. It is easier to follow if the > > unwinding of state changes are local to the function. > I think so. It is easier to understand if the charge and uncharge funct= ions > are grouped together in the same function. > >=20 > > With the proposed simplification by Willy > > Acked-by: Michal Hocko >=20 > Thank for the ack. However, I am a bit confused about what you mean by > simplification. There is another linux-next patch that changes the cond= ition > for mem_cgroup_charge() to >=20 > -=A0=A0=A0=A0=A0=A0 if (!huge) { > +=A0=A0=A0=A0=A0=A0 if (!huge && !page_is_secretmem(page)) { > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 error =3D mem_cgroup_char= ge(page, current->mm, gfp); >=20 > That is the main reason why I introduced the boolean variable as I don'= t > want to call the external page_is_secretmem() function twice. See earlier emails to Mike suggesting that the accounting of secretmem here is wrong.