Re: RDMA/rpma + fsdax(ext4) was broken since 36f30e486d

[Jason Gunthorpe]

On Fri, Aug 27, 2021 at 09:42:21AM -0700, Dan Williams wrote:
> On Fri, Aug 27, 2021 at 6:05 AM Li, Zhijian <lizhijian@xxxxxxxxxxxxxx> wrote:
> >
> >
> > on 2021/8/27 20:10, Jason Gunthorpe wrote:
> > > On Fri, Aug 27, 2021 at 08:15:40AM +0000, lizhijian@xxxxxxxxxxx wrote:
> > >> i looked over the change-log of hmm_vma_handle_pte(), and found that before
> > >> 4055062 ("mm/hmm: add missing call to hmm_pte_need_fault in HMM_PFN_SPECIAL handling")
> > >>
> > >> hmm_vma_handle_pte() will not check pte_special(pte) if pte_devmap(pte) is true.
> > >>
> > >> when we reached
> > >> "if (pte_special(pte) && !is_zero_pfn(pte_pfn(pte))) {"
> > >> the pte have already presented and its pte's flag already fulfilled the request flags.
> > >>
> > >>
> > >> My question is that
> > >> Per https://01.org/blogs/dave/2020/linux-consumption-x86-page-table-bits,
> > >> pte_devmap(pte) and pte_special(pte) could be both true in fsdax user case, right ?
> > > How? what code creates that?
> > >
> > > I see:
> > >
> > > insert_pfn():
> > >       /* Ok, finally just insert the thing.. */
> > >       if (pfn_t_devmap(pfn))
> > >               entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
> > >       else
> > >               entry = pte_mkspecial(pfn_t_pte(pfn, prot));
> > >
> > > So what code path ends up setting both bits?
> >
> >   pte_mkdevmap() will set both _PAGE_SPECIAL | PAGE_DEVMAP
> >
> >   395 static inline pte_t pte_mkdevmap(pte_t pte)
> >   396 {
> >   397         return pte_set_flags(pte, _PAGE_SPECIAL|_PAGE_DEVMAP);
> >   398 }
> 
> I can't recall why _PAGE_SPECIAL is there. I'll take a look, but I
> think setting _PAGE_SPECIAL in pte_mkdevmap() is overkill.

This is my feeling too, but every arch does it, so hmm should check
it, at least for now as a stable fix

devmap has a struct page so it should be refcounted inside the VMA and
that is the main thing that PAGE_SPECIAL disabled, AFAICR..

The only places where pte_special are used that I wonder if are OK for
devmap have to do with CPU cache maintenance

vm_normal_page(), hmm_vma_handle_pte(), gup_pte_range() all look OK to
drop the special bit

Jason