Re: [RFC PATCH v2 1/6] KVM: gmem: Truncate pages on punch hole

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 21, 2023, Sean Christopherson wrote:
> On Thu, Sep 21, 2023, isaku.yamahata@intel.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@intel.com>
> > 
> > Although kvm_gmem_punch_hole() keeps all pages in mapping on punching hole,
> > it's common expectation that pages are truncated.  Truncate pages on
> > punching hole.  As page contents can be encrypted, avoid zeroing partial
> > folio by refusing partial punch hole.
> > 
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > ---
> >  virt/kvm/guest_mem.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/virt/kvm/guest_mem.c b/virt/kvm/guest_mem.c
> > index a819367434e9..01fb4ca861d0 100644
> > --- a/virt/kvm/guest_mem.c
> > +++ b/virt/kvm/guest_mem.c
> > @@ -130,22 +130,32 @@ static void kvm_gmem_invalidate_end(struct kvm_gmem *gmem, pgoff_t start,
> >  static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> >  {
> >  	struct list_head *gmem_list = &inode->i_mapping->private_list;
> > +	struct address_space *mapping  = inode->i_mapping;
> >  	pgoff_t start = offset >> PAGE_SHIFT;
> >  	pgoff_t end = (offset + len) >> PAGE_SHIFT;
> >  	struct kvm_gmem *gmem;
> >  
> > +	/*
> > +	 * punch hole may result in zeroing partial area.  As pages can be
> > +	 * encrypted, prohibit zeroing partial area.
> > +	 */
> > +	if (offset & ~PAGE_MASK || len & ~PAGE_MASK)
> > +		return -EINVAL;
> 
> This should be unnecessary, kvm_gmem_fallocate() does
> 
> 	if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
> 		return -EINVAL;
> 
> before invoking kvm_gmem_punch_hole().  If that's not working, i.e. your test
> fails, then that code needs to be fixed.  I'll run your test to double-check,
> but AFAICT this is unnecesary.

I confirmed that the testcase passes without the extra checks.  Just to close the
loop, what prompted adding more checks to kvm_gmem_punch_hole()?