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

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Date:   Fri, 27 Aug 2021 09:10:34 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "lizhijian@...itsu.com" <lizhijian@...itsu.com>
Cc:     "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 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?

Jason