Re: [PATCH] memcontrol: ensure memcg acquired by id is properly set up

[Johannes Weiner]

+CC Shakeel

On Wed, Aug 23, 2023 at 03:54:30PM -0700, Nhat Pham wrote:
> From: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> In the eviction recency check, we attempt to retrieve the memcg to which
> the folio belonged when it was evicted, by the memcg id stored in the
> shadow entry. However, there is a chance that the retrieved memcg is not
> the original memcg that has been killed, but a new one which happens to
> have the same id.
> 
> This is a somewhat unfortunate, but acceptable and rare inaccuracy in
> the heuristics. However, if we retrieve this new memcg between its
> allocation and when it is properly attached to the memcg hierarchy, we
> could run into the following NULL pointer exception during the memcg
> hierarchy traversal done in mem_cgroup_get_nr_swap_pages():
> 
> [ 155757.793456] BUG: kernel NULL pointer dereference, address: 00000000000000c0
> [ 155757.807568] #PF: supervisor read access in kernel mode
> [ 155757.818024] #PF: error_code(0x0000) - not-present page
> [ 155757.828482] PGD 401f77067 P4D 401f77067 PUD 401f76067 PMD 0
> [ 155757.839985] Oops: 0000 [#1] SMP
> [ 155757.887870] RIP: 0010:mem_cgroup_get_nr_swap_pages+0x3d/0xb0
> [ 155757.899377] Code: 29 19 4a 02 48 39 f9 74 63 48 8b 97 c0 00 00 00 48 8b b7 58 02 00 00 48 2b b7 c0 01 00 00 48 39 f0 48 0f 4d c6 48 39 d1 74 42 <48> 8b b2 c0 00 00 00 48 8b ba 58 02 00 00 48 2b ba c0 01 00 00 48
> [ 155757.937125] RSP: 0018:ffffc9002ecdfbc8 EFLAGS: 00010286
> [ 155757.947755] RAX: 00000000003a3b1c RBX: 000007ffffffffff RCX: ffff888280183000
> [ 155757.962202] RDX: 0000000000000000 RSI: 0007ffffffffffff RDI: ffff888bbc2d1000
> [ 155757.976648] RBP: 0000000000000001 R08: 000000000000000b R09: ffff888ad9cedba0
> [ 155757.991094] R10: ffffea0039c07900 R11: 0000000000000010 R12: ffff888b23a7b000
> [ 155758.005540] R13: 0000000000000000 R14: ffff888bbc2d1000 R15: 000007ffffc71354
> [ 155758.019991] FS:  00007f6234c68640(0000) GS:ffff88903f9c0000(0000) knlGS:0000000000000000
> [ 155758.036356] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 155758.048023] CR2: 00000000000000c0 CR3: 0000000a83eb8004 CR4: 00000000007706e0
> [ 155758.062473] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 155758.076924] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 155758.091376] PKRU: 55555554
> [ 155758.096957] Call Trace:
> [ 155758.102016]  <TASK>
> [ 155758.106502]  ? __die+0x78/0xc0
> [ 155758.112793]  ? page_fault_oops+0x286/0x380
> [ 155758.121175]  ? exc_page_fault+0x5d/0x110
> [ 155758.129209]  ? asm_exc_page_fault+0x22/0x30
> [ 155758.137763]  ? mem_cgroup_get_nr_swap_pages+0x3d/0xb0
> [ 155758.148060]  workingset_test_recent+0xda/0x1b0
> [ 155758.157133]  workingset_refault+0xca/0x1e0
> [ 155758.165508]  filemap_add_folio+0x4d/0x70
> [ 155758.173538]  page_cache_ra_unbounded+0xed/0x190
> [ 155758.182919]  page_cache_sync_ra+0xd6/0x1e0
> [ 155758.191738]  filemap_read+0x68d/0xdf0
> [ 155758.199495]  ? mlx5e_napi_poll+0x123/0x940
> [ 155758.207981]  ? __napi_schedule+0x55/0x90
> [ 155758.216095]  __x64_sys_pread64+0x1d6/0x2c0
> [ 155758.224601]  do_syscall_64+0x3d/0x80
> [ 155758.232058]  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> [ 155758.242473] RIP: 0033:0x7f62c29153b5
> [ 155758.249938] Code: e8 48 89 75 f0 89 7d f8 48 89 4d e0 e8 b4 e6 f7 ff 41 89 c0 4c 8b 55 e0 48 8b 55 e8 48 8b 75 f0 8b 7d f8 b8 11 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 33 44 89 c7 48 89 45 f8 e8 e7 e6 f7 ff 48 8b
> [ 155758.288005] RSP: 002b:00007f6234c5ffd0 EFLAGS: 00000293 ORIG_RAX: 0000000000000011
> [ 155758.303474] RAX: ffffffffffffffda RBX: 00007f628c4e70c0 RCX: 00007f62c29153b5
> [ 155758.318075] RDX: 000000000003c041 RSI: 00007f61d2986000 RDI: 0000000000000076
> [ 155758.332678] RBP: 00007f6234c5fff0 R08: 0000000000000000 R09: 0000000064d5230c
> [ 155758.347452] R10: 000000000027d450 R11: 0000000000000293 R12: 000000000003c041
> [ 155758.362044] R13: 00007f61d2986000 R14: 00007f629e11b060 R15: 000000000027d450
> [ 155758.376661]  </TASK>
> 
> This patch fixes the issue by moving the memcg's id publication from the
> alloc stage to online stage, ensuring that any memcg acquired via id
> must be connected to the memcg tree.
> 
> Fixes: f78dfc7b77d5 ("workingset: fix confusion around eviction vs refault container")
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Co-developed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  mm/memcontrol.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index cf57fe9318d5..6d67a3a81cd9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5326,7 +5326,6 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>  	INIT_LIST_HEAD(&memcg->deferred_split_queue.split_queue);
>  	memcg->deferred_split_queue.split_queue_len = 0;
>  #endif
> -	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
>  	lru_gen_init_memcg(memcg);
>  	return memcg;
>  fail:
> @@ -5398,14 +5397,27 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
>  	if (alloc_shrinker_info(memcg))
>  		goto offline_kmem;
>  
> -	/* Online state pins memcg ID, memcg ID pins CSS */
> -	refcount_set(&memcg->id.ref, 1);
> -	css_get(css);
> -
>  	if (unlikely(mem_cgroup_is_root(memcg)))
>  		queue_delayed_work(system_unbound_wq, &stats_flush_dwork,
>  				   FLUSH_TIME);
>  	lru_gen_online_memcg(memcg);
> +
> +	/* Online state pins memcg ID, memcg ID pins CSS */
> +	refcount_set(&memcg->id.ref, 1);
> +	css_get(css);
> +
> +	/*
> +	 * Ensure mem_cgroup_from_id() works once we're fully online.
> +	 *
> +	 * We could do this earlier and require callers to filter with
> +	 * css_tryget_online(). But right now there are no users that
> +	 * need earlier access, and the workingset code relies on the
> +	 * cgroup tree linkage (mem_cgroup_get_nr_swap_pages()). So
> +	 * publish it here at the end of onlining. This matches the
> +	 * regular ID destruction during offlining.
> +	 */
> +	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
> +
>  	return 0;
>  offline_kmem:
>  	memcg_offline_kmem(memcg);
> -- 
> 2.34.1
>