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

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Message-ID: <CAPcyv4ho-42iZB3W5ypfwj-2=+v6rRUCcwE4ntPXyDPgFjzp7g@mail.gmail.com>
Date:   Fri, 27 Aug 2021 09:42:21 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Li, Zhijian" <lizhijian@...fujitsu.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        "lizhijian@...itsu.com" <lizhijian@...itsu.com>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        Yishai Hadas <yishaih@...dia.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "yangx.jy@...itsu.com" <yangx.jy@...itsu.com>
Subject: Re: RDMA/rpma + fsdax(ext4) was broken since 36f30e486d

On Fri, Aug 27, 2021 at 6:05 AM Li, Zhijian <lizhijian@...fujitsu.com> wrote:
>
>
> on 2021/8/27 20:10, Jason Gunthorpe wrote:
> > On Fri, Aug 27, 2021 at 08:15:40AM +0000, lizhijian@...itsu.com 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.