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
