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

[Vincent Donnefort <vdonnefort@google.com>]

On Tue, May 21, 2024 at 11:18:41AM +0200, David Hildenbrand wrote:
> On 21.05.24 11:06, Vincent Donnefort wrote:
> > On Tue, May 21, 2024 at 10:25:43AM +0200, David Hildenbrand wrote:
> > > 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.
> > 
> > I have a patch ready to use this path from the memory map tracing! I can either
> > send it once this one is picked-up or you can add it to your series?
> 
> Whatever works for you! To debug, it would be good if you could send me the
> patch and simple instructions on how to test it (do we have a selftest as
> well?).

Of course, I'll share something with you today! It includes an update to the
selftest to make sure we check the padding with the zero-page.

> 
> > 
> > > 
> > > [...]
> > > 
> > > > > @@ -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() ?
> > 
> > I meant this page_count() here. As a quick test, I removed that check (also fixed
> > the vm_flags above) and the zero-page was properly mapped!
> 
> That's weird and might indicate another issue.
> 
> The refcount of the shared zeropage should be initialized to 1, just like
> for any other reserved pages
> (mm/mm_init.c:__init_single_page()->init_page_count())
> 
> Hm ...
> 
> -- 
> Cheers,
> 
> David / dhildenb
>