Re: [Qemu-devel] [PATCH v5 4/6] memfd: add hugetlb support

[Daniel P. Berrange]

On Mon, Oct 23, 2017 at 07:27:31AM -0400, Marc-André Lureau wrote:
> Hi
> 
> ----- Original Message -----
> > On Mon, Oct 23, 2017 at 10:59:08AM +0100, Marc-André Lureau wrote:
> > > Linux commit 749df87bd7bee5a79cef073f5d032ddb2b211de8 (v4.14-rc1)
> > > added a new flag MFD_HUGETLB to memfd_create() that specify the file
> > > to be created resides in the hugetlbfs filesystem.  This is the
> > > generic hugetlbfs filesystem not associated with any specific mount
> > > point.
> > 
> > How do you know and/or control what size huge pages are used,
> > when the platform supports many sizes ?
> 
> From linux commit message:
> "As with other system calls that request hugetlbfs backed pages, there is
> the ability to encode huge page size in the flag arguments."
> 
> I didn't add this option to memfd backend, as I don't know how generally
> useful that is, and if there is already a similar option in qemu (probably
> not?). It could be easily added later.

For the existing  memory-backend-file, libvirt controls hugepage size
based on what's requested in guest XML. So if we were to support the
new memory-backend-memfd as an alternative, I think we need to be able
to choose huge page size for that too.

Regards,
Daniel
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