Re: [PATCH 12/19] mm: memcontrol: convert anon and file-thp to new mem_cgroup_charge() API

[Qian Cai]

> On May 8, 2020, at 2:30 PM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> 
> With the page->mapping requirement gone from memcg, we can charge anon
> and file-thp pages in one single step, right after they're allocated.
> 
> This removes two out of three API calls - especially the tricky commit
> step that needed to happen at just the right time between when the
> page is "set up" and when it's "published" - somewhat vague and fluid
> concepts that varied by page type. All we need is a freshly allocated
> page and a memcg context to charge.
> 
> v2: prevent double charges on pre-allocated hugepages in khugepaged
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
> include/linux/mm.h      |  4 +---
> kernel/events/uprobes.c | 11 +++--------
> mm/filemap.c            |  2 +-
> mm/huge_memory.c        |  9 +++------
> mm/khugepaged.c         | 35 ++++++++++-------------------------
> mm/memory.c             | 36 ++++++++++--------------------------
> mm/migrate.c            |  5 +----
> mm/swapfile.c           |  6 +-----
> mm/userfaultfd.c        |  5 +----
> 9 files changed, 31 insertions(+), 82 deletions(-)
[]
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> 
> @@ -1198,10 +1193,11 @@ static void collapse_huge_page(struct mm_struct *mm,
> out_up_write:
> 	up_write(&mm->mmap_sem);
> out_nolock:
> +	if (*hpage)
> +		mem_cgroup_uncharge(*hpage);
> 	trace_mm_collapse_huge_page(mm, isolated, result);
> 	return;
> out:
> -	mem_cgroup_cancel_charge(new_page, memcg);
> 	goto out_up_write;
> }
[]

Some memory pressure will crash this new code. It looks like somewhat racy.

if (!page->mem_cgroup)

where page == NULL in mem_cgroup_uncharge().

[ 2244.414421][  T726] BUG: Kernel NULL pointer dereference on read at 0x0000002c
[ 2244.414454][  T726] Faulting instruction address: 0xc0000000004f7e44
[ 2244.414467][  T726] Oops: Kernel access of bad area, sig: 11 [#1]
[ 2244.414488][  T726] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=256 DEBUG_PAGEALLOC NUMA PowerNV
[ 2244.414501][  T726] Modules linked in: brd ext4 crc16 mbcache jbd2 loop kvm_hv kvm ip_tables x_tables xfs sd_mod bnx2x ahci tg3 libahci libphy mdio libata firmware_class dm_mirror dm_region_hash dm_log dm_mod
[ 2244.414556][  T726] CPU: 11 PID: 726 Comm: khugepaged Not tainted 5.7.0-rc5-next-20200512+ #8
[ 2244.414579][  T726] NIP:  c0000000004f7e44 LR: c0000000004df95c CTR: c0000000001c1400
[ 2244.414600][  T726] REGS: c000001a2398f6e0 TRAP: 0300   Not tainted  (5.7.0-rc5-next-20200512+)
[ 2244.414630][  T726] MSR:  9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 24000244  XER: 20040000
[ 2244.414656][  T726] CFAR: c0000000004df958 DAR: 000000000000002c DSISR: 40000000 IRQMASK: 0 
[ 2244.414656][  T726] GPR00: c0000000004df95c c000001a2398f970 c00000000168a700 fffffffffffffff4 
[ 2244.414656][  T726] GPR04: ffffffffffffffff c000000000bd0980 0000000000000005 0000000000000080 
[ 2244.414656][  T726] GPR08: 0000001ffc030000 0000000000000001 0000000000000000 c00000000152bb58 
[ 2244.414656][  T726] GPR12: 0000000024000222 c000001fffff5680 c0000001d818ce00 c0000001d818cd00 
[ 2244.414656][  T726] GPR16: 0000000000000000 c000001a2398fce0 fe7fffffffffefff fffffffffffffe7f 
[ 2244.414656][  T726] GPR20: c000201320aa53c8 000000000000001e 0000000000000017 c00020047636b868 
[ 2244.414656][  T726] GPR24: 0000000000000000 0000000000000000 c000000001756080 c000001a2398fce0 
[ 2244.414656][  T726] GPR28: c000001a2398fa20 00007ffeeda00000 c000200f28547928 c000200f28547880 
[ 2244.414865][  T726] NIP [c0000000004f7e44] mem_cgroup_uncharge+0x34/0xb0
mem_cgroup_uncharge at mm/memcontrol.c:6563
[ 2244.414895][  T726] LR [c0000000004df95c] collapse_huge_page+0x24c/0x1000
collapse_huge_page at mm/khugepaged.c:1197
[ 2244.414924][  T726] Call Trace:
[ 2244.414940][  T726] [c000001a2398f970] [0000000000000001] 0x1 (unreliable)
[ 2244.414970][  T726] [c000001a2398f9c0] [c0000000004df814] collapse_huge_page+0x104/0x1000
collapse_huge_page at mm/khugepaged.c:1064 (discriminator 10)
[ 2244.414991][  T726] [c000001a2398faf0] [c0000000004e0f84] khugepaged_scan_pmd+0x874/0xc70
[ 2244.415021][  T726] [c000001a2398fbf0] [c0000000004e2a90] khugepaged+0x900/0x1920
[ 2244.415043][  T726] [c000001a2398fdb0] [c000000000155aa4] kthread+0x1c4/0x1d0
[ 2244.415075][  T726] [c000001a2398fe20] [c00000000000cb28] ret_from_kernel_thread+0x5c/0x74
[ 2244.415095][  T726] Instruction dump:
[ 2244.415113][  T726] 384228f0 7c0802a6 60000000 f821ffb1 e92d0c70 f9210048 39200000 3d22ffec 
[ 2244.415146][  T726] 3929f9f4 81290000 2f890000 409d0048 <e9230038> 2fa90000 419e003c 7c0802a6 
[ 2244.415181][  T726] ---[ end trace 3488eb8818913a26 ]---