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

[Waiman Long <longman@redhat.com>]

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@redhat.com>
---
 mm/filemap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 5c9d564317a5..aa0e0fb04670 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -835,6 +835,7 @@ noinline int __add_to_page_cache_locked(struct page *page,
 	XA_STATE(xas, &mapping->i_pages, offset);
 	int huge = PageHuge(page);
 	int error;
+	bool charged = false;
 
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(PageSwapBacked(page), page);
@@ -848,6 +849,7 @@ noinline int __add_to_page_cache_locked(struct page *page,
 		error = mem_cgroup_charge(page, current->mm, gfp);
 		if (error)
 			goto error;
+		charged = true;
 	}
 
 	gfp &= GFP_RECLAIM_MASK;
@@ -896,6 +898,8 @@ noinline int __add_to_page_cache_locked(struct page *page,
 
 	if (xas_error(&xas)) {
 		error = xas_error(&xas);
+		if (charged)
+			mem_cgroup_uncharge(page);
 		goto error;
 	}
 
-- 
2.18.1