netdev - Re: [PATCH bpf-next v7 32/34] bpf: eliminate rlimit-based memory accounting infra for bpf maps

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Message-ID: <20201121025227.dypweojhaz7elwb3@ast-mbp>
Date:   Fri, 20 Nov 2020 18:52:27 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Roman Gushchin <guro@...com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        netdev@...r.kernel.org, andrii@...nel.org,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH bpf-next v7 32/34] bpf: eliminate rlimit-based memory
 accounting infra for bpf maps

On Thu, Nov 19, 2020 at 09:37:52AM -0800, Roman Gushchin wrote:
>  static void bpf_map_put_uref(struct bpf_map *map)
> @@ -619,7 +562,7 @@ static void bpf_map_show_fdinfo(struct seq_file *m, struct file *filp)
>  		   "value_size:\t%u\n"
>  		   "max_entries:\t%u\n"
>  		   "map_flags:\t%#x\n"
> -		   "memlock:\t%llu\n"
> +		   "memlock:\t%llu\n" /* deprecated */
>  		   "map_id:\t%u\n"
>  		   "frozen:\t%u\n",
>  		   map->map_type,
> @@ -627,7 +570,7 @@ static void bpf_map_show_fdinfo(struct seq_file *m, struct file *filp)
>  		   map->value_size,
>  		   map->max_entries,
>  		   map->map_flags,
> -		   map->memory.pages * 1ULL << PAGE_SHIFT,
> +		   0LLU,

The set looks great to me overall, but above change is problematic.
There are tools out there that read this value.
Returning zero might cause oncall alarms to trigger.
I think we can be more accurate here.
Instead of zero the kernel can return
round_up(max_entries * round_up(key_size + value_size, 8), PAGE_SIZE)
It's not the same as before, but at least the numbers won't suddenly
go to zero and comparison between maps is still relevant.
Of course we can introduce a page size calculating callback per map type,
but imo that would be overkill. These monitoring tools don't care about
precise number, but rather about relative value and growth from one
version of the application to another.

If Daniel doesn't find other issues this can be fixed in the follow up.