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Date:   Wed, 2 May 2018 09:40:26 -0400
From:   "John Stoffel" <john@stoffel.org>
To:     Mike Snitzer <snitzer@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, eric.dumazet@gmail.com,
        mst@redhat.com, netdev@vger.kernel.org, jasowang@redhat.com,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org,
        dm-devel@redhat.com, Mikulas Patocka <mpatocka@redhat.com>,
        Babka <vbabka@suse.cz>,
        virtualization@lists.linux-foundation.org,
        David Miller <davem@davemloft.net>, edumazet@google.com
Subject: Re: [dm-devel] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM
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>>>>> "Mike" == Mike Snitzer <snitzer@redhat.com> writes:

Mike> On Tue, May 01 2018 at  8:36pm -0400,
Mike> Andrew Morton <akpm@linux-foundation.org> wrote:

>> On Tue, 24 Apr 2018 12:33:01 -0400 (EDT) Mikulas Patocka <mpatocka@redhat.com> wrote:
>> 
>> > 
>> > 
>> > On Tue, 24 Apr 2018, Michal Hocko wrote:
>> > 
>> > > On Tue 24-04-18 11:30:40, Mikulas Patocka wrote:
>> > > > 
>> > > > 
>> > > > On Tue, 24 Apr 2018, Michal Hocko wrote:
>> > > > 
>> > > > > On Mon 23-04-18 20:25:15, Mikulas Patocka wrote:
>> > > > > 
>> > > > > > Fixing __vmalloc code 
>> > > > > > is easy and it doesn't require cooperation with maintainers.
>> > > > > 
>> > > > > But it is a hack against the intention of the scope api.
>> > > > 
>> > > > It is not!
>> > > 
>> > > This discussion simply doesn't make much sense it seems. The scope API
>> > > is to document the scope of the reclaim recursion critical section. That
>> > > certainly is not a utility function like vmalloc.
>> > 
>> > That 15-line __vmalloc bugfix doesn't prevent you (or any other kernel 
>> > developer) from converting the code to the scope API. You make nonsensical 
>> > excuses.
>> > 
>> 
>> Fun thread!
>> 
>> Winding back to the original problem, I'd state it as
>> 
>> - Caller uses kvmalloc() but passes the address into vmalloc-naive
>> DMA API and
>> 
>> - Caller uses kvmalloc() but passes the address into kfree()
>> 
>> Yes?

Mike> I think so.

>> If so, then...
>> 
>> Is there a way in which, in the kvmalloc-called-kmalloc path, we can
>> tag the slab-allocated memory with a "this memory was allocated with
>> kvmalloc()" flag?  I *think* there's extra per-object storage available
>> with suitable slab/slub debugging options?  Perhaps we could steal one
>> bit from the redzone, dunno.
>> 
>> If so then we can
>> 
>> a) set that flag in kvmalloc() if the kmalloc() call succeeded
>> 
>> b) check for that flag in the DMA code, WARN if it is set.
>> 
>> c) in kvfree(), clear that flag before calling kfree()
>> 
>> d) in kfree(), check for that flag and go WARN() if set.
>> 
>> So both potential bugs are detected all the time, dependent upon
>> CONFIG_SLUB_DEBUG (and perhaps other slub config options).

Mike> Thanks Andrew, definitely the most sane proposal I've seen to resolve
Mike> this.

Cuts to the heart of the issue I think, and seems pretty sane.  Should
the WARN be rate limited as well?

John