Re: [PATCH] mm/filemap: Adding missing mem_cgroup_uncharge() to __add_to_page_cache_locked()

[Johannes Weiner]

On Mon, Jan 25, 2021 at 05:41:18PM +0100, 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@xxxxxxxxxx>
> > > > > 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.

FWIW, khugepaged does a similar type of page recycling, where when one
virtual area fails to collapse, the scanner caches the physical page
and tries to reuse it for the next area. See the mem_cgroup_uncharge()
call in collapse_huge_page().

While it's nice to be able to leave the uncharge to the free call, in
reuse cases like this I don't think it's a problem to have a matching
uncharge.