linux-ext4 - Re: [PATCH 02/13] pci/p2pdma: Don't initialise page refcount to one

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Message-ID: <20240629212851.GA1484889@bhelgaas>
Date: Sat, 29 Jun 2024 16:28:51 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Alistair Popple <apopple@...dia.com>
Cc: dan.j.williams@...el.com, vishal.l.verma@...el.com,
	dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com,
	jack@...e.cz, jgg@...pe.ca, catalin.marinas@....com,
	will@...nel.org, mpe@...erman.id.au, npiggin@...il.com,
	dave.hansen@...ux.intel.com, ira.weiny@...el.com,
	willy@...radead.org, djwong@...nel.org, tytso@....edu,
	linmiaohe@...wei.com, david@...hat.com, peterx@...hat.com,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
	nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
	jhubbard@...dia.com, hch@....de, david@...morbit.com
Subject: Re: [PATCH 02/13] pci/p2pdma: Don't initialise page refcount to one

On Thu, Jun 27, 2024 at 10:54:17AM +1000, Alistair Popple wrote:
> The reference counts for ZONE_DEVICE private pages should be
> initialised by the driver when the page is actually allocated by the
> driver allocator, not when they are first created. This is currently
> the case for MEMORY_DEVICE_PRIVATE and MEMORY_DEVICE_COHERENT pages
> but not MEMORY_DEVICE_PCI_P2PDMA pages so fix that up.

If you tag the subject line with PCI, please run "git log --oneline
drivers/pci/p2pdma.c" and make yours look like previous ones
("PCI/P2PDMA").

Also recast it to say something semantically useful about what it
*does*, not what it *doesn't* do.  Maybe something about initializing
the refcount where the page is allocated?  Especially since the only
p2pdma.c change here is to "set_page_count(..., 1)", which looks like
exactly the opposite of "don't initialize refcount to one".

> Signed-off-by: Alistair Popple <apopple@...dia.com>
> ---
>  drivers/pci/p2pdma.c | 2 ++
>  mm/memremap.c        | 8 ++++----
>  mm/mm_init.c         | 4 +++-
>  3 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 4f47a13..1e9ea32 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -128,6 +128,8 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
>  		goto out;
>  	}
>  
> +	set_page_count(virt_to_page(kaddr), 1);
> +
>  	/*
>  	 * vm_insert_page() can sleep, so a reference is taken to mapping
>  	 * such that rcu_read_unlock() can be done before inserting the
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 40d4547..caccbd8 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -488,15 +488,15 @@ void free_zone_device_folio(struct folio *folio)
>  	folio->mapping = NULL;
>  	folio->page.pgmap->ops->page_free(folio_page(folio, 0));
>  
> -	if (folio->page.pgmap->type != MEMORY_DEVICE_PRIVATE &&
> -	    folio->page.pgmap->type != MEMORY_DEVICE_COHERENT)
> +	if (folio->page.pgmap->type == MEMORY_DEVICE_PRIVATE ||
> +	    folio->page.pgmap->type == MEMORY_DEVICE_COHERENT)
> +		put_dev_pagemap(folio->page.pgmap);
> +	else if (folio->page.pgmap->type != MEMORY_DEVICE_PCI_P2PDMA)
>  		/*
>  		 * Reset the refcount to 1 to prepare for handing out the page
>  		 * again.
>  		 */
>  		folio_set_count(folio, 1);
> -	else
> -		put_dev_pagemap(folio->page.pgmap);
>  }
>  
>  void zone_device_page_init(struct page *page)
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 3ec0493..b7e1599 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -6,6 +6,7 @@
>   * Author Mel Gorman <mel@....ul.ie>
>   *
>   */
> +#include "linux/memremap.h"
>  #include <linux/kernel.h>
>  #include <linux/init.h>
>  #include <linux/kobject.h>
> @@ -1014,7 +1015,8 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>  	 * which will set the page count to 1 when allocating the page.
>  	 */
>  	if (pgmap->type == MEMORY_DEVICE_PRIVATE ||
> -	    pgmap->type == MEMORY_DEVICE_COHERENT)
> +	    pgmap->type == MEMORY_DEVICE_COHERENT ||
> +	    pgmap->type == MEMORY_DEVICE_PCI_P2PDMA)
>  		set_page_count(page, 0);
>  }
>  
> -- 
> git-series 0.9.1