Re: [RFC 0/2] vduse: add support for networking devices

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On Fri, Apr 21, 2023 at 2:39 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> On Thu, Apr 20, 2023 at 12:34:06PM +0800, Jason Wang wrote:
> > > 3. Coredump:
> > >   In order to be able to perform post-mortem analysis, DPDK
> > >   Vhost library marks pages used for vrings and descriptors
> > >   buffers as MADV_DODUMP using madvise(). However with
> > >   VDUSE it fails with -EINVAL. My understanding is that we
> > >   set VM_DONTEXPAND flag to the VMAs and madvise's
> > >   MADV_DODUMP fails if it is present. I'm not sure to
> > >   understand why madvise would prevent MADV_DODUMP if
> > >   VM_DONTEXPAND is set. Any thoughts?
> >
> > Adding Peter who may know the answer.
> I don't.. but I had a quick look, it seems that VM_DONTEXPAND was kind of
> reused (and I'm not sure whether it's an abuse or not so far..) to
> represent device driver pages since removal of VM_RESERVED:

Interesting, but there're indeed cases where VM_DONTEXPAND is used by
non-driver codes. The pages mapped by VDUSE are not device driver
pages but the IOTLB and virtqueue.

Btw the definition of VM_SPECIAL doesn't say anything related to dump:

 * Special vmas that are non-mergable, non-mlock()able.

> But I think that change at least breaks hugetlb once so there's the
> explicit hugetlb check to recover that behavior back:

This seems similar to the case of VDUSE.


> Thanks,
> --
> Peter Xu

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