Re: [PATCH -next 1/7] mm: huge_memory: make __do_huge_pmd_anonymous_page() to take a folio

[Kefeng Wang]

On 2023/1/13 22:25, Matthew Wilcox wrote:
> On Thu, Jan 12, 2023 at 04:30:00PM +0800, Kefeng Wang wrote:
> > Let's __do_huge_pmd_anonymous_page() take a folio and convert related
> > functions to use folios.
>
> No, this is actively wrong!  Andrew, please drop this patch.
>
> If we want to support folio sizes larger than PMD size (and I think we
> do), we need to be able to specify precisely which page in the folio
> is to be stored at this PTE.  The *interface* must remain struct page.
> We can convert from page to folio within the function, but we *MUST NOT*
> go the other way.

Got it,

> >   static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
> > -			struct page *page, gfp_t gfp)
> > +			struct folio *folio, gfp_t gfp)
> >   {
> >   	struct vm_area_struct *vma = vmf->vma;
> > +	struct page *page = &folio->page;
>
> ... ie this is bad and wrong.
>
> > @@ -834,7 +835,7 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> >   		count_vm_event(THP_FAULT_FALLBACK);
> >   		return VM_FAULT_FALLBACK;
> >   	}
> > -	return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp);
> > +	return __do_huge_pmd_anonymous_page(vmf, folio, gfp);
> >   }
> >   
> >   static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>
> A reasonable person might ask "But Matthew, you allocated a folio here,
> then you're converting back to a struct page to call
> __do_huge_pmd_anonymous_page() so isn't this a sensible patch?"

and this is why I change the parameter from page to folio(no need to go 
back and forth between page and folio),
> And I would say "still no".  This is a question of interfaces, and
> even though __do_huge_pmd_anonymous_page() is static and has precisely
> one caller today that always allocates a folio of precisely PMD size,
> I suspect it will need to be more visible in the future and the
> conversion of the interface from page to folio misleads people.
ok, will keep page for __do_huge_pmd_anonymous_page().
>