linux-kernel - Re: [PATCH] mm/hmm: bypass devmap pte when all pfn requested flags are fulfilled

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Date:   Fri, 27 Aug 2021 13:28:52 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Li Zhijian <lizhijian@...fujitsu.com>
Cc:     linux-mm@...ck.org, linux-rdma@...r.kernel.org,
        akpm@...ux-foundation.org, jglisse@...hat.com, yishaih@...dia.com,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm/hmm: bypass devmap pte when all pfn requested flags
 are fulfilled

On Fri, Aug 27, 2021 at 10:45:00PM +0800, Li Zhijian wrote:
> Previously, we noticed the one rpma example was failed[1] since 36f30e486d,
> where it will use ODP feature to do RDMA WRITE between fsdax files.
> 
> After digging into the code, we found hmm_vma_handle_pte() will still
> return EFAULT even though all the its requesting flags has been
> fulfilled. That's because a DAX page will be marked as
> (_PAGE_SPECIAL | PAGE_DEVMAP) by pte_mkdevmap().
> 
> [1]: https://github.com/pmem/rpma/issues/1142
> 
> CC: stable@...r.kernel.org
> Signed-off-by: Li Zhijian <lizhijian@...fujitsu.com>

You need to add a 

Fixes: 405506274922 ("mm/hmm: add missing call to hmm_pte_need_fault in HMM_PFN_SPECIAL handling")

> diff --git a/mm/hmm.c b/mm/hmm.c
> index fad6be2bf072..4766bdefb6c3 100644
> +++ b/mm/hmm.c
> @@ -294,6 +294,12 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>  	if (required_fault)
>  		goto fault;
>  
> +	/*
> +	 * just bypass devmap pte such as DAX page when all pfn requested
> +	 * flags(pfn_req_flags) are fulfilled.
> +	 */
> +	if (pte_devmap(pte))
> +		goto out;

I liked your ealier version better where this was added to the
pte_special test - logically this is about disambiguating the
pte_special and the devmap case as they are different things.

Jason