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

[Waiman Long]

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.

Cheers,
Longman