[RFC PATCH 2/9] x86/sgx: Do not naturally align MAP_FIXED address

[Xing, Cedric]

> From: Jarkko Sakkinen [mailto:jarkko.sakkinen at linux.intel.com]
> Sent: Thursday, June 13, 2019 6:48 AM
> 
> On Thu, Jun 06, 2019 at 06:37:10PM +0300, Jarkko Sakkinen wrote:
> > On Wed, Jun 05, 2019 at 01:14:04PM -0700, Andy Lutomirski wrote:
> > >
> > >
> > > > On Jun 5, 2019, at 8:17 AM, Jarkko Sakkinen
> <jarkko.sakkinen at linux.intel.com> wrote:
> > > >
> > > >> On Tue, Jun 04, 2019 at 10:10:22PM +0000, Xing, Cedric wrote:
> > > >> A bit off topic here. This mmap()/mprotect() discussion reminds
> > > >> me a question (guess for Jarkko): Now that
> > > >> vma->vm_file->private_data keeps a pointer to the enclave, why do
> we store it again in vma->vm_private?
> > > >> It isn't a big deal but non-NULL vm_private does prevent
> > > >> mprotect() from merging adjacent VMAs.
> > > >
> > > > Same semantics as with a regular mmap i.e. you can close the file
> > > > and still use the mapping.
> > > >
> > > >
> > >
> > > The file should be properly refcounted — vm_file should not go away
> while it’s mapped.
> 
> mm already takes care of that so vm_file does not go away. Still we need
> an internal refcount for enclaves to synchronize with the swapper. In
> summary nothing needs to be done.

I don't get this. The swapper takes a read lock on mm->mmap_sem, which locks the vma, which in turn reference counts vma->vm_file. Why is the internal refcount still needed? 

> 
> > Right, makes sense. It is easy one to change essentially just removing
> > internal refcount from sgx_encl and using file for the same. I'll
> > update this to my tree along with the changes to remove LKM/ACPI bits
> ASAP.
> 
> /Jarkko