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Date:   Thu, 3 May 2018 13:40:48 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     John Stoffel <john@stoffel.org>
cc:     Andrew@stoffel.org, dm-devel@redhat.com, eric.dumazet@gmail.com,
        mst@redhat.com, netdev@vger.kernel.org, jasowang@redhat.com,
        Randy Dunlap <rdunlap@infradead.org>,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
        Hocko <mhocko@kernel.org>,
        James Bottomley <James.Bottomley@HansenPartnership.com>,
        Michal@stoffel.org, edumazet@google.com, linux-mm@kvack.org,
        David Rientjes <rientjes@google.com>,
        Morton <akpm@linux-foundation.org>,
        virtualization@lists.linux-foundation.org,
        David Miller <davem@davemloft.net>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [dm-devel] [PATCH v5] fault-injection: introduce kvmalloc fallback
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On Wed, 2 May 2018, John Stoffel wrote:

> You miss my point, which is that there's no explanation of what the
> difference is between SLAB and SLUB and which I should choose.  The
> same goes here.  If the KConfig option doesn't give useful info, it's
> useless.

So what, we could write explamantion of that option.

> >> Now I also think that Linus has the right idea to not just sprinkle 
> >> BUG_ONs into the code, just dump and oops and keep going if you can.  
> >> If it's a filesystem or a device, turn it read only so that people 
> >> notice right away.
> 
> Mikulas> This vmalloc fallback is similar to
> Mikulas> CONFIG_DEBUG_KOBJECT_RELEASE.  CONFIG_DEBUG_KOBJECT_RELEASE
> Mikulas> changes the behavior of kobject_put in order to cause
> Mikulas> deliberate crashes (that wouldn't happen otherwise) in
> Mikulas> drivers that misuse kobject_put. In the same sense, we want
> Mikulas> to cause deliberate crashes (that wouldn't happen otherwise)
> Mikulas> in drivers that misuse kvmalloc.
> 
> Mikulas> The crashes will only happen in debugging kernels, not in
> Mikulas> production kernels.
> 
> Says you.  What about people or distros that enable it
> unconditionally?  They're going to get all kinds of reports and then
> turn it off again.  Crashing the system isn't the answer here.  

I've made that kvmalloc bug too (in the function 
dm_integrity_free_journal_scatterlist). I'd much rather like if the kernel 
crashed (because then - I would fix the bug). The kernel didn't crash and 
the bug sneaked into the official linux tree, where may be causing random 
crashes for other users.

Mikulas