linux-kernel - Re: [v5 05/15] mm/migrate_device: handle partially mapped folios during collection

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Message-ID: <20250918164213.2866702-1-clm@meta.com>
Date: Thu, 18 Sep 2025 09:42:07 -0700
From: Chris Mason <clm@...a.com>
To: Balbir Singh <balbirs@...dia.com>
CC: Chris Mason <clm@...a.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <dri-devel@...ts.freedesktop.org>,
        Andrew Morton
	<akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, Zi Yan
	<ziy@...dia.com>,
        Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim
	<rakie.kim@...com>,
        Byungchul Park <byungchul@...com>, Gregory Price
	<gourry@...rry.net>,
        Ying Huang <ying.huang@...ux.alibaba.com>,
        "Alistair
 Popple" <apopple@...dia.com>,
        Oscar Salvador <osalvador@...e.de>,
        "Lorenzo
 Stoakes" <lorenzo.stoakes@...cle.com>,
        Baolin Wang
	<baolin.wang@...ux.alibaba.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain
	<dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Lyude Paul
	<lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>,
        David Airlie
	<airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Ralph Campbell
	<rcampbell@...dia.com>,
        Mika Penttilä
	<mpenttil@...hat.com>,
        Matthew Brost <matthew.brost@...el.com>,
        "Francois
 Dugast" <francois.dugast@...el.com>
Subject: Re: [v5 05/15] mm/migrate_device: handle partially mapped folios during collection

On Mon,  8 Sep 2025 10:04:38 +1000 Balbir Singh <balbirs@...dia.com> wrote:

> Extend migrate_vma_collect_pmd() to handle partially mapped large
> folios that require splitting before migration can proceed.
> 
> During PTE walk in the collection phase, if a large folio is only
> partially mapped in the migration range, it must be split to ensure
> the folio is correctly migrated.
> 
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
>  mm/migrate_device.c | 94 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 94 insertions(+)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index abd9f6850db6..f45ef182287d 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -54,6 +54,53 @@ static int migrate_vma_collect_hole(unsigned long start,
>  	return 0;
>  }
>  
> +/**
> + * migrate_vma_split_folio() - Helper function to split a THP folio
> + * @folio: the folio to split
> + * @fault_page: struct page associated with the fault if any
> + *
> + * Returns 0 on success
> + */
> +static int migrate_vma_split_folio(struct folio *folio,
> +				   struct page *fault_page)
> +{
> +	int ret;
> +	struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL;
> +	struct folio *new_fault_folio = NULL;
> +
> +	if (folio != fault_folio) {
> +		folio_get(folio);
> +		folio_lock(folio);
> +	}

Can fault_folio ever be non-null and different from folio? Apologies for
not knowing the lock ordering rules but this jumps out.

> +
> +	ret = split_folio(folio);
> +	if (ret) {
> +		if (folio != fault_folio) {
> +			folio_unlock(folio);
> +			folio_put(folio);
> +		}
> +		return ret;
> +	}
> +
> +	new_fault_folio = fault_page ? page_folio(fault_page) : NULL;
> +
> +	/*
> +	 * Ensure the lock is held on the correct
> +	 * folio after the split
> +	 */
> +	if (!new_fault_folio) {
> +		folio_unlock(folio);
> +		folio_put(folio);
> +	} else if (folio != new_fault_folio) {
> +		folio_get(new_fault_folio);
> +		folio_lock(new_fault_folio);
> +		folio_unlock(folio);
> +		folio_put(folio);
> +	}

Same question here, do we need trylocks?

-chris