linux-kernel - Re: [v5 01/15] mm/zone_device: support large zone device private folios

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Message-ID: <20250918122813.1440093-1-clm@meta.com>
Date: Thu, 18 Sep 2025 05:27:58 -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 01/15] mm/zone_device: support large zone device private folios


[ apologies for resending this, debugging sendmail ]

Hi Balbir,

On Mon,  8 Sep 2025 10:04:34 +1000 Balbir Singh <balbirs@...dia.com> wrote:
> Add routines to support allocation of large order zone device folios
> and helper functions for zone device folios, to check if a folio is
> device private and helpers for setting zone device data.
> 
> When large folios are used, the existing page_free() callback in
> pgmap is called when the folio is freed, this is true for both
> PAGE_SIZE and higher order pages.
> 
> Zone device private large folios do not support deferred split and
> scan like normal THP folios.

[ ... ]

> diff --git a/mm/memremap.c b/mm/memremap.c
> index 46cb1b0b6f72..66f9186b5500 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -453,11 +452,15 @@ void free_zone_device_folio(struct folio *folio)
>  
>  	switch (pgmap->type) {
>  	case MEMORY_DEVICE_PRIVATE:
> +		percpu_ref_put_many(&folio->pgmap->ref, nr);

Here we're dropping nr refs

> +		pgmap->ops->page_free(&folio->page);
> +		folio->page.mapping = NULL;
> +		break;
>  	case MEMORY_DEVICE_COHERENT:
>  		if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->page_free))
>  			break;
> -		pgmap->ops->page_free(folio_page(folio, 0));
> -		put_dev_pagemap(pgmap);
> +		pgmap->ops->page_free(&folio->page);
> +		percpu_ref_put(&folio->pgmap->ref);

Here we're dropping one ref?

>  		break;
>  
>  	case MEMORY_DEVICE_GENERIC:
> @@ -480,14 +483,23 @@ void free_zone_device_folio(struct folio *folio)
>  	}
>  }
>  
> -void zone_device_page_init(struct page *page)
> +void zone_device_folio_init(struct folio *folio, unsigned int order)
>  {
> +	struct page *page = folio_page(folio, 0);
> +
> +	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
> +
>  	/*
>  	 * Drivers shouldn't be allocating pages after calling
>  	 * memunmap_pages().
>  	 */
> -	WARN_ON_ONCE(!percpu_ref_tryget_live(&page_pgmap(page)->ref));
> -	set_page_count(page, 1);
> +	WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << order));

Here we always bump by 1 << order

I hesitate to send this one because I don't know the code at all, but the
AI review prompts keep flagging this apparent refcount mismatch, and it looks
real to me.

Are the differences in refcount handling inside free_zone_device_folio()
intentional?

-chris