linux-kernel - Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG

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Message-ID: <alpine.LRH.2.02.1804201635180.25408@file01.intranet.prod.int.rdu2.redhat.com>
Date:   Fri, 20 Apr 2018 16:54:53 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     David Miller <davem@...emloft.net>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        eric.dumazet@...il.com, edumazet@...gle.com,
        bhutchings@...arflare.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, mst@...hat.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org, dm-devel@...hat.com,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM

On Fri, 20 Apr 2018, Michal Hocko wrote:

> On Thu 19-04-18 12:12:38, Mikulas Patocka wrote:
> [...]
> > From: Mikulas Patocka <mpatocka@...hat.com>
> > Subject: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM
> > 
> > The kvmalloc function tries to use kmalloc and falls back to vmalloc if
> > kmalloc fails.
> > 
> > Unfortunatelly, some kernel code has bugs - it uses kvmalloc and then
> > uses DMA-API on the returned memory or frees it with kfree. Such bugs were
> > found in the virtio-net driver, dm-integrity or RHEL7 powerpc-specific
> > code.
> > 
> > These bugs are hard to reproduce because vmalloc falls back to kmalloc
> > only if memory is fragmented.
> > 
> > In order to detect these bugs reliably I submit this patch that changes
> > kvmalloc to always use vmalloc if CONFIG_DEBUG_VM is turned on.
> 
> No way. This is just wrong! First of all, you will explode most likely
> on many allocations of small sizes. Second, CONFIG_DEBUG_VM tends to be
> enabled quite often.

You're an evil person who doesn't want to fix bugs.

You refused to fix vmalloc(GFP_NOIO) misbehavior a year ago (did you make 
some progress with it since that time?) and you refuse to fix kvmalloc 
misuses.

I tried this patch on text-only virtual machine and /proc/vmallocinfo 
shows 614kB more memory. I tried it on a desktop machine with the chrome 
browser open and /proc/vmallocinfo space is increased by 7MB. So no - this 
won't exhaust memory and kill the machine.

Arguing that this increases memory consumption is as bogus as arguing that 
CONFIG_LOCKDEP increses memory consumption. No one is forcing you to 
enable CONFIG_LOCKDEP and no one is forcing you to enable this kvmalloc 
test too.

Mikulas