linux-kernel - Re: Linux 5.9-rc7 / VmallocTotal wrongly reported

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Date:   Thu, 1 Oct 2020 12:19:57 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Bastian Bittorf <bb@....de>, Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 5.9-rc7 / VmallocTotal wrongly reported

Adding Vlastimil, Roman and the kernel mailing list to the cc.

Vlastimil, Roman - this looks like a slab regression. And while others
have touched slab in this merge window, you guys did so more than
most.. Comments?

On Wed, Sep 30, 2020 at 11:55 PM Bastian Bittorf <bb@....de> wrote:
>
> Since 5.9-rc1 i can see false values, booting with qemu (128mb RAM):
>
> / # free
>               total        used        free      shared  buff/cache available
> Mem:         122176 18014398506028288      114848           0 3461024     3568096

that looks like a negative 64-bit number shifted down by 10 bits (bytes -> kB).

> / # cat /proc/meminfo | grep ^V
> VmallocTotal:   34359738367 kB

.. but this part is normal. VMALLOC_TOTAL is the VM space for vmalloc
areas, and it's large on a 64-bit machine.

And afaik, it hasn't changed in a long time.

So I think there's something else confusing 'free'.

I think it's this:

> / # cat /proc/meminfo
> MemAvailable:    3617184 kB
..
> Slab:           10321920 kB
> SReclaimable:    3506176 kB
> SUnreclaim:      6815744 kB

because those slab reclaimable/unreclaimable numbers look completely
bogus. You have more unreclaimable slab space than you have memory.

Hmm. I'm not seeing anything similar. And your config looks sane, with
the main difference I see wrt SLAB being that you have

# CONFIG_SLAB_MERGE_DEFAULT is not set

but there's nothing wrong with that either.

It would be lovely if you could bisect it, but maybe Vlastimil or
Roman will go "Ahh, obviously.."

                 Linus