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

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Date:   Fri, 27 Aug 2021 21:05:21 +0800
From:   "Li, Zhijian" <lizhijian@...fujitsu.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        "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 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 }

below is a calltrace example

[  400.728559] Call Trace:
[  400.731595] dump_stack+0x6d/0x8b
[  400.735536] insert_pfn+0x16c/0x180
[  400.739596] __vm_insert_mixed+0x84/0xc0
[  400.744144] dax_iomap_pte_fault+0x845/0x870
[  400.749089] ext4_dax_huge_fault+0x171/0x1e0
[  400.754096] __do_fault+0x31/0xe0
[  400.758090]  ? pmd_devmap_trans_unstable+0x37/0x90
[  400.763541] handle_mm_fault+0x11b1/0x1680
[  400.768260] exc_page_fault+0x2f4/0x570
[  400.772788]  ? asm_exc_page_fault+0x8/0x30
[  400.777539]  asm_exc_page_fault+0x1e/0x30


So is my previous change reasonable ?

Thanks

Zhijian