Re: [PATCH v1 1/2] mm/memory: cleanly support zeropage in vm_insert_page*(), vm_map_pages*() and vmf_insert_mixed()

[David Hildenbrand]

On 17.05.24 17:07, Vincent Donnefort wrote:
> Hi David,
>
> [...]
>
> > -static int validate_page_before_insert(struct page *page)
> > +static bool vm_mixed_zeropage_allowed(struct vm_area_struct *vma)
> > +{
> > +	VM_WARN_ON_ONCE(vma->vm_flags & VM_PFNMAP);
> > +	/*
> > +	 * Whoever wants to forbid the zeropage after some zeropages
> > +	 * might already have been mapped has to scan the page tables and
> > +	 * bail out on any zeropages. Zeropages in COW mappings can
> > +	 * be unshared using FAULT_FLAG_UNSHARE faults.
> > +	 */
> > +	if (mm_forbids_zeropage(vma->vm_mm))
> > +		return false;
> > +	/* zeropages in COW mappings are common and unproblematic. */
> > +	if (is_cow_mapping(vma->vm_flags))
> > +		return true;
> > +	/* Mappings that do not allow for writable PTEs are unproblematic. */
> > +	if (!(vma->vm_flags & (VM_WRITE | VM_MAYWRITE)))
> > +		return false;
>
> Shouldn't we return true here?

Indeed, thanks! I wish we would have user in the tree already that could 
exercise that code path.

[...]

> > @@ -2043,7 +2085,7 @@ static int insert_page_in_batch_locked(struct vm_area_struct *vma, pte_t *pte,
> >   
> >   	if (!page_count(page))
> >   		return -EINVAL;
>
> This test here prevents inserting the zero-page.

You mean the existing page_count() check? or the (wrong) vma->vm_flags 
check in vm_mixed_zeropage_allowed() ?

--
Cheers,

David / dhildenb