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Date: Tue, 24 Apr 2018 07:04:23 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
Subject: Re: [PATCH v3] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG
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To: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@kernel.org>, Matthew Wilcox <willy@infradead.org>, David Miller <davem@davemloft.net>, Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, eric.dumazet@gmail.com, edumazet@google.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, mst@redhat.com, jasowang@redhat.com, virtualization@lists.linux-foundation.org, dm-devel@redhat.com, Vlastimil Babka <vbabka@suse.cz>



On Mon, 23 Apr 2018, David Rientjes wrote:

> On Mon, 23 Apr 2018, Mikulas Patocka wrote:
> 
> > 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 kvmalloc falls back to vmalloc
> > only if memory is fragmented.
> > 
> > In order to detect these bugs reliably I submit this patch that changes
> > kvmalloc to fall back to vmalloc with 1/2 probability if CONFIG_DEBUG_SG
> > is turned on. CONFIG_DEBUG_SG is used, because it makes the DMA API layer
> > verify the addresses passed to it, and so the user will get a reliable
> > stacktrace.
> 
> Why not just do it unconditionally?  Sounds better than "50% of the time 
> this will catch bugs".

Because kmalloc (with slub_debug) detects buffer overflows better than 
vmalloc. vmalloc detects buffer overflows only at a page boundary. This is 
intended for debugging kernels and debugging kernels should detect as many 
bugs as possible.

Mikulas