Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

[Kirill A . Shutemov]

On Thu, Sep 22, 2022 at 07:49:18PM +0000, Sean Christopherson wrote:
> On Thu, Sep 22, 2022, Wang, Wei W wrote:
> > On Thursday, September 15, 2022 10:29 PM, Chao Peng wrote:
> > > +int inaccessible_get_pfn(struct file *file, pgoff_t offset, pfn_t *pfn,
> > > +			 int *order)
> > 
> > Better to remove "order" from this interface?
> 
> Hard 'no'.
> 
> > Some callers only need to get pfn, and no need to bother with
> > defining and inputting something unused. For callers who need the "order",
> > can easily get it via thp_order(pfn_to_page(pfn)) on their own.
> 
> That requires (a) assuming the pfn is backed by struct page, and (b) assuming the
> struct page is a transparent huge page.  That might be true for the current
> implementation, but it most certainly will not always be true.
> 
> KVM originally did things like this, where there was dedicated code for THP vs.
> HugeTLB, and it was a mess.  The goal here is very much to avoid repeating those
> mistakes.  Have the backing store _tell_ KVM how big the mapping is, don't force
> KVM to rediscover the info on its own.

I guess we can allow order pointer to be NULL to cover caller that don't
need to know the order. Is it useful?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov