Re: [PATCH 09/19] mm: New follow_pfnmap API

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 16, 2024 at 04:12:24PM -0700, Sean Christopherson wrote:
> On Fri, Aug 09, 2024, Peter Xu wrote:
> > Introduce a pair of APIs to follow pfn mappings to get entry information.
> > It's very similar to what follow_pte() does before, but different in that
> > it recognizes huge pfn mappings.
> 
> ...
> 
> > +int follow_pfnmap_start(struct follow_pfnmap_args *args);
> > +void follow_pfnmap_end(struct follow_pfnmap_args *args);
> 
> I find the start+end() terminology to be unintuitive.  E.g. I had to look at the
> implementation to understand why KVM invoke fixup_user_fault() if follow_pfnmap_start()
> failed.
> 
> What about follow_pfnmap_and_lock()?  And then maybe follow_pfnmap_unlock()?
> Though that second one reads a little weird.

If to go with the _lock() I tend to drop "and" to follow_pfnmap_[un]lock().
However looks like David preferred me keeping the name, so we don't reach a
quorum yet.  I'm happy to change the name as long as we have enough votes..

> 
> > + * Return: zero on success, -ve otherwise.
> 
> ve?

This one came from the old follow_pte() and I kept it. I only knew this
after search: a short way to write "negative" (while positive is "+ve").

Doesn't look like something productive.. I'll spell it out in the next
version.

> 
> > +int follow_pfnmap_start(struct follow_pfnmap_args *args)
> > +{
> > +	struct vm_area_struct *vma = args->vma;
> > +	unsigned long address = args->address;
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	spinlock_t *lock;
> > +	pgd_t *pgdp;
> > +	p4d_t *p4dp, p4d;
> > +	pud_t *pudp, pud;
> > +	pmd_t *pmdp, pmd;
> > +	pte_t *ptep, pte;
> > +
> > +	pfnmap_lockdep_assert(vma);
> > +
> > +	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
> > +		goto out;
> > +
> > +	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> > +		goto out;
> 
> Why use goto intead of simply?
> 
> 		return -EINVAL;
> 
> That's relevant because I think the cases where no PxE is found should return
> -ENOENT, not -EINVAL.  E.g. if the caller doesn't precheck, then it can bail
> immediately on EINVAL, but know that it's worth trying to fault-in the pfn on
> ENOENT. 

I tend to avoid changing the retval in this series to make the goal of this
patchset simple.

One issue is I _think_ there's one ioctl() that will rely on this retval:

      acrn_dev_ioctl ->
        acrn_vm_memseg_map ->
          acrn_vm_ram_map ->
            follow_pfnmap_start

So we may want to try check with people to not break it..

Thanks,

-- 
Peter Xu