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

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Message-ID: <20201001195610.GC2706729@carbon.DHCP.thefacebook.com>
Date:   Thu, 1 Oct 2020 12:56:10 -0700
From:   Roman Gushchin <guro@...com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Bastian Bittorf <bb@....de>, Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 5.9-rc7 / VmallocTotal wrongly reported

On Thu, Oct 01, 2020 at 12:19:57PM -0700, Linus Torvalds wrote:
> 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?

Thank you, Linus!

I think it's a regression from my slab controller changes, and I have a theory.
Bastian, can you, please, share your config?

I will come up with a patch soon.

Thanks!

> 
> 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