Re: [PATCH] memcg, kmem: do not fail __GFP_NOFAIL charges

[Shakeel Butt]

On Fri, Sep 6, 2019 at 5:56 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> Thomas has noticed the following NULL ptr dereference when using cgroup
> v1 kmem limit:
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
> PGD 0
> P4D 0
> Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 3 PID: 16923 Comm: gtk-update-icon Not tainted 4.19.51 #42
> Hardware name: Gigabyte Technology Co., Ltd. Z97X-Gaming G1/Z97X-Gaming G1, BIOS F9 07/31/2015
> RIP: 0010:create_empty_buffers+0x24/0x100
> Code: cd 0f 1f 44 00 00 0f 1f 44 00 00 41 54 49 89 d4 ba 01 00 00 00 55 53 48 89 fb e8 97 fe ff ff 48 89 c5 48 89 c2 eb 03 48 89 ca <48> 8b 4a 08 4c 09 22 48 85 c9 75 f1 48 89 6a 08 48 8b 43 18 48 8d
> RSP: 0018:ffff927ac1b37bf8 EFLAGS: 00010286
> RAX: 0000000000000000 RBX: fffff2d4429fd740 RCX: 0000000100097149
> RDX: 0000000000000000 RSI: 0000000000000082 RDI: ffff9075a99fbe00
> RBP: 0000000000000000 R08: fffff2d440949cc8 R09: 00000000000960c0
> R10: 0000000000000002 R11: 0000000000000000 R12: 0000000000000000
> R13: ffff907601f18360 R14: 0000000000002000 R15: 0000000000001000
> FS:  00007fb55b288bc0(0000) GS:ffff90761f8c0000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000008 CR3: 000000007aebc002 CR4: 00000000001606e0
> Call Trace:
>  create_page_buffers+0x4d/0x60
>  __block_write_begin_int+0x8e/0x5a0
>  ? ext4_inode_attach_jinode.part.82+0xb0/0xb0
>  ? jbd2__journal_start+0xd7/0x1f0
>  ext4_da_write_begin+0x112/0x3d0
>  generic_perform_write+0xf1/0x1b0
>  ? file_update_time+0x70/0x140
>  __generic_file_write_iter+0x141/0x1a0
>  ext4_file_write_iter+0xef/0x3b0
>  __vfs_write+0x17e/0x1e0
>  vfs_write+0xa5/0x1a0
>  ksys_write+0x57/0xd0
>  do_syscall_64+0x55/0x160
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
>  Tetsuo then noticed that this is because the __memcg_kmem_charge_memcg
>  fails __GFP_NOFAIL charge when the kmem limit is reached. This is a
>  wrong behavior because nofail allocations are not allowed to fail.
>  Normal charge path simply forces the charge even if that means to cross
>  the limit. Kmem accounting should be doing the same.
>
> Reported-by: Thomas Lindroth <thomas.lindroth@xxxxxxxxx>
> Debugged-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: stable
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

I wonder what has changed since
<http://lkml.kernel.org/r/20180525185501.82098-1-shakeelb@xxxxxxxxxx/>.

> ---
>  mm/memcontrol.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9ec5e12486a7..e18108b2b786 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2821,6 +2821,16 @@ int __memcg_kmem_charge_memcg(struct page *page, gfp_t gfp, int order,
>
>         if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) &&
>             !page_counter_try_charge(&memcg->kmem, nr_pages, &counter)) {
> +
> +               /*
> +                * Enforce __GFP_NOFAIL allocation because callers are not
> +                * prepared to see failures and likely do not have any failure
> +                * handling code.
> +                */
> +               if (gfp & __GFP_NOFAIL) {
> +                       page_counter_charge(&memcg->kmem, nr_pages);
> +                       return 0;
> +               }
>                 cancel_charge(memcg, nr_pages);
>                 return -ENOMEM;
>         }
> --
> 2.20.1
>