linux-kernel - Re: [PATCH 4/7] mm: Centralize & improve oom reporting in show

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Date:   Tue, 28 Nov 2023 11:07:18 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH 4/7] mm: Centralize & improve oom reporting in show_mem.c

On Wed 22-11-23 18:25:09, Kent Overstreet wrote:
[...]
> 00177 Shrinkers:
> 00177 super_cache_scan: objects: 127
> 00177 super_cache_scan: objects: 106
> 00177 jbd2_journal_shrink_scan: objects: 32
> 00177 ext4_es_scan: objects: 32
> 00177 bch2_btree_cache_scan: objects: 8
> 00177   nr nodes:          24
> 00177   nr dirty:          0
> 00177   cannibalize lock:  0000000000000000
> 00177
> 00177 super_cache_scan: objects: 8
> 00177 super_cache_scan: objects: 1

It would be really great to provide an example on how these numbers are
useful for the oom evaluation.

[...]
> @@ -423,4 +426,21 @@ void __show_mem(unsigned int filter, nodemask_t *nodemask, int max_zone_idx)
>  #ifdef CONFIG_MEMORY_FAILURE
>  	printk("%lu pages hwpoisoned\n", atomic_long_read(&num_poisoned_pages));
>  #endif
> +
> +	buf = kmalloc(4096, GFP_ATOMIC);

I really do not think we want to allow allocations from the OOM context.
Is there any reason why this cannot be a statically allocated buffer?

> +	if (buf) {
> +		struct seq_buf s;
> +
> +		printk("Unreclaimable slab info:\n");
> +		seq_buf_init(&s, buf, 4096);
> +		dump_unreclaimable_slab(&s);
> +		printk("%s", seq_buf_str(&s));
> +
> +		printk("Shrinkers:\n");
> +		seq_buf_init(&s, buf, 4096);
> +		shrinkers_to_text(&s);
> +		printk("%s", seq_buf_str(&s));
> +
> +		kfree(buf);
> +	}
>  }
-- 
Michal Hocko
SUSE Labs