linux-kernel - Re: [RFC PATCH 0/5] madvise MADV

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Date:   Fri, 31 Jul 2020 13:55:07 -0400
From:   Steven Sistare <steven.sistare@...cle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org, mhocko@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        arnd@...db.de, keescook@...omium.org, gerg@...ux-m68k.org,
        ktkhai@...tuozzo.com, christian.brauner@...ntu.com,
        peterz@...radead.org, esyr@...hat.com, christian@...lner.me,
        areber@...hat.com, cyphar@...har.com
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

On 7/31/2020 1:48 PM, Jason Gunthorpe wrote:
> On Fri, Jul 31, 2020 at 01:15:34PM -0400, Steven Sistare wrote:
>> On 7/31/2020 12:56 PM, Jason Gunthorpe wrote:
>>> On Fri, Jul 31, 2020 at 12:11:52PM -0400, Steven Sistare wrote:
>>>>> Your preservation-across-exec use-case might or might not need the
>>>>> VMA to be mapped at the same address.  
>>>>
>>>> It does.  qemu registers memory with vfio which remembers the va's in kernel
>>>> metadata for the device.
>>>
>>> Once the memory is registered with vfio the VA doesn't matter, vfio
>>> will keep the iommu pointing at the same physical pages no matter
>>> where they are mapped.
>>
>> Yes, but there are other code paths that compute and use offsets between va and the
>> base va.  Mapping at a different va in the new process breaks vfio; I have tried it.
> 
> Maybe you could fix vfio instead of having this adventure, if vfio is
> the only motivation.

Maybe.  We still need to preserve an anonymous segment, though.  MADV_DOEXEC, or mshare,
or something else.  And I think the ability to preserve memory containing pointers to itself
is an interesting use case, though not ours.

- Steve