Re: [RFC 00/10] fs/dax: Fix FS DAX page reference counts

Dan Williams
2024-04-12T17:32:51Z

Jason Gunthorpe wrote:
> On Fri, Apr 12, 2024 at 04:55:31PM +1000, Alistair Popple wrote:
>
> > Ok, I think I found the dragons you were talking about earlier for
> > device-dax. I completely broke that because as you've already pointed
> > out pmd_trans_huge() won't filter out DAX pages. That's fine for FS DAX
> > (because the pages are essentially normal pages now anyway), but we
> > don't have a PMD equivalent of vm_normal_page() which leads to all sorts
> > of issues for DEVDAX.
>
> What about vm_normal_page() depends on the radix level ?
>
> Doesn't DEVDAX memory have struct page too?

Yes.

> > So I will probably have to add something like that unless we only need
> > to support large (pmd/pud) mappings of DEVDAX pages on systems with
> > CONFIG_ARCH_HAS_PTE_SPECIAL in which case I guess we could just filter
> > based on pte_special().
>
> pte_special should only be used by memory without a struct page, is
> that what DEVDAX is?

Right, I don't think pte_special is applicable for any DAX pages.