linux-kernel - Re: [PATCH] mm/hmm: Do not fault in device private pages owned by the caller

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Message-ID: <aJVDGSUzuKgy0PtK@lstrano-desk.jf.intel.com>
Date: Thu, 7 Aug 2025 17:21:45 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Francois Dugast <francois.dugast@...el.com>
CC: <balbirs@...dia.com>, <airlied@...il.com>, <akpm@...ux-foundation.org>,
	<apopple@...dia.com>, <baohua@...nel.org>, <baolin.wang@...ux.alibaba.com>,
	<dakr@...nel.org>, <david@...hat.com>, <donettom@...ux.ibm.com>,
	<jane.chu@...cle.com>, <jglisse@...hat.com>, <kherbst@...hat.com>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>, <lyude@...hat.com>,
	<peterx@...hat.com>, <ryan.roberts@....com>, <shuah@...nel.org>,
	<simona@...ll.ch>, <wangkefeng.wang@...wei.com>, <willy@...radead.org>,
	<ziy@...dia.com>
Subject: Re: [PATCH] mm/hmm: Do not fault in device private pages owned by
 the caller

On Tue, Jul 22, 2025 at 09:34:45PM +0200, Francois Dugast wrote:
> When the PMD swap entry is device private and owned by the caller,
> skip the range faulting and instead just set the correct HMM PFNs.
> This is similar to the logic for PTEs in hmm_vma_handle_pte().
> 
> For now, each hmm_pfns[i] entry is populated as it is currently done
> in hmm_vma_handle_pmd() but this might not be necessary. A follow-up
> optimization could be to make use of the order and skip populating
> subsequent PFNs.
> 
> Signed-off-by: Francois Dugast <francois.dugast@...el.com>
> ---
>  mm/hmm.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index f2415b4b2cdd..63ec1b18a656 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -355,6 +355,31 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>  	}
>  
>  	if (!pmd_present(pmd)) {
> +		swp_entry_t entry = pmd_to_swp_entry(pmd);
> +
> +		/*
> +		 * Don't fault in device private pages owned by the caller,
> +		 * just report the PFNs.
> +		 */
> +		if (is_device_private_entry(entry) &&
> +		    pfn_swap_entry_folio(entry)->pgmap->owner ==
> +		    range->dev_private_owner) {
> +			unsigned long cpu_flags = HMM_PFN_VALID |
> +				hmm_pfn_flags_order(PMD_SHIFT - PAGE_SHIFT);
> +			unsigned long pfn = swp_offset_pfn(entry);
> +			unsigned long i;
> +
> +			if (is_writable_device_private_entry(entry))
> +				cpu_flags |= HMM_PFN_WRITE;
> +
> +			for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
> +				hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS;
> +				hmm_pfns[i] |= pfn | cpu_flags;
> +			}
> +
> +			return 0;
> +		}
> +
>  		if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
>  			return -EFAULT;

I think here if this is a is_device_private_entry(entry), we need to
call hmm_vma_fault.

Matt

>  		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
> -- 
> 2.43.0
>