Date:   Mon, 25 Jan 2021 18:29:21 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/filemap: Adding missing mem_cgroup_uncharge() to
 __add_to_page_cache_locked()

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:
> > > > > > > > 
> > > > > > > >    [ 1570.068330] page dumped because: VM_BUG_ON_PAGE(page_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: 00010286
> > > > > > > >    [ 1570.068379] RAX: 0000000000000000 RBX: ffffea00260ddd00 RCX: 0000000000000027
> > > > > > > >    [ 1570.068382] RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffff88907ebe05a8
> > > > > > > >    [ 1570.068384] RBP: ffffea00260ddd00 R08: ffffed120fd7c0b6 R09: ffffed120fd7c0b6
> > > > > > > >    [ 1570.068386] R10: ffff88907ebe05ab R11: ffffed120fd7c0b5 R12: ffffea00260ddd38
> > > > > > > >    [ 1570.068389] R13: ffff8889a4116000 R14: ffff8889a4116000 R15: 0000000000000001
> > > > > > > >    [ 1570.068391] FS:  00007ff039638680(0000) GS:ffff88907ea00000(0000) knlGS:0000000000000000
> > > > > > > >    [ 1570.068394] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > > > > >    [ 1570.068396] CR2: 00007f36f354cc20 CR3: 00000008a0126006 CR4: 00000000007706e0
> > > > > > > >    [ 1570.068398] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > > > > >    [ 1570.068400] DR3: 0000000000000000 DR6: 00000000fffe0ff0 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+0x895/0x2e10 [cachefiles]
> > > > > > > >    [ 1570.068524]  __fscache_read_or_alloc_pages+0x6c0/0xa00 [fscache]
> > > > > > > >    [ 1570.068540]  __nfs_readpages_from_fscache+0x16d/0x630 [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+0x290/0x1710
> > > > > > > >    [ 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+0x44/0xa9
> > > > > > > >    [ 1570.068930] RIP: 0033:0x7ff039135595
> > > > > > > > 
> > > > > > > > Before that commit, there was a try_charge() and commit_charge()
> > > > > > > > in __add_to_page_cache_locked(). These 2 separated charge functions
> > > > > > > > were replaced by a single mem_cgroup_charge(). However, it forgot
> > > > > > > > to add a matching mem_cgroup_uncharge() when the xarray insertion
> > > > > > > > failed with the page released back to the pool. Fix this by adding a
> > > > > > > > mem_cgroup_uncharge() call when insertion error happens.
> > > > > > > > 
> > > > > > > > Fixes: 3fea5a499d57 ("mm: memcontrol: convert page cache to a new mem_cgroup_charge() API")
> > > > > > > > Signed-off-by: Waiman Long <longman@...hat.com>
> > > > > > > OK, this is indeed a subtle bug. The patch aimed at simplifying the
> > > > > > > charge lifetime so that users do not really have to think about when to
> > > > > > > uncharge as that happens when the page is freed. fscache somehow breaks
> > > > > > > that assumption because it doesn't free up pages but it keeps some of
> > > > > > > them in the cache.
> > > > > > > 
> > > > > > > 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 only instance
> > > > > > > of the problem? Would it make more sense to explicitly handle 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 produce 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 called
> > > > '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.
> > > 
> > > > I don't understand the memcg code at all, so I have no useful feedback
> > > > on what you're saying other than this.
> > > Well the memcg accounting rules after the rework should have simplified
> > > the API usage for most users. You will get memory charged when it is
> > > used and it will go away when the page is freed. If a page is not really
> > > 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 additional
> > > burden on those special cases. I am not really sure what is the right
> > > way here myself but considering there might be other similar cases like
> > > that I would lean towards special casing where the pool is implemented.
> > > 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 functions
> are grouped together in the same function.
> > 
> > With the proposed simplification by Willy
> > Acked-by: Michal Hocko <mhocko@...e.com>
> 
> 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 condition
> for mem_cgroup_charge() to
> 
> -       if (!huge) {
> +       if (!huge && !page_is_secretmem(page)) {
>                 error = mem_cgroup_charge(page, current->mm, gfp);
> 
> 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.

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