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

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Message-Id: <20250722130721.9169d564eeeb28807e18635a@linux-foundation.org>
Date: Tue, 22 Jul 2025 13:07:21 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Francois Dugast <francois.dugast@...el.com>
Cc: balbirs@...dia.com, airlied@...il.com, 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, matthew.brost@...el.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, 22 Jul 2025 21:34:45 +0200 Francois Dugast <francois.dugast@...el.com> 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().

Please always tell us why a patch does something, not only what it does.

> 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.

I infer from this paragraph that this patch is a performance
optimization?  Have its effects been measured?

> --- 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.
> +		 */

Similarly, this tells us "what" it does, which is fairly obvious from
the code itself.  What is not obvious from the code is the "why".

> +		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;
>  		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);