linux-kernel - Re: [PATCH 02/13] PCI/P2PDMA: Fix the warning condition in p2pmem_alloc

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Message-ID: <r227azjk4mhat6pxfqodcbnlstqfjndsmkeebm2e6lz2yjiudo@uuy3ageaiv5s>
Date: Thu, 8 Jan 2026 14:28:18 +1100
From: Alistair Popple <apopple@...dia.com>
To: Hou Tao <houtao@...weicloud.com>
Cc: linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, 
	linux-mm@...ck.org, linux-nvme@...ts.infradead.org, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Logan Gunthorpe <logang@...tatee.com>, 
	Leon Romanovsky <leonro@...dia.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Tejun Heo <tj@...nel.org>, "Rafael J . Wysocki" <rafael@...nel.org>, 
	Danilo Krummrich <dakr@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...nel.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>, 
	Sagi Grimberg <sagi@...mberg.me>, houtao1@...wei.com
Subject: Re: [PATCH 02/13] PCI/P2PDMA: Fix the warning condition in
 p2pmem_alloc_mmap()

On 2025-12-20 at 15:04 +1100, Hou Tao <houtao@...weicloud.com> wrote...
> From: Hou Tao <houtao1@...wei.com>
> 
> Commit b7e282378773 has already changed the initial page refcount of
> p2pdma page from one to zero, however, in p2pmem_alloc_mmap() it uses
> "VM_WARN_ON_ONCE_PAGE(!page_ref_count(page))" to assert the initial page
> refcount should not be zero and the following will be reported when
> CONFIG_DEBUG_VM is enabled:
> 
>  page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x380400000
>  flags: 0x20000000002000(reserved|node=0|zone=4)
>  raw: 0020000000002000 ff1100015e3ab440 0000000000000000 0000000000000000
>  raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
>  page dumped because: VM_WARN_ON_ONCE_PAGE(!page_ref_count(page))
>  ------------[ cut here ]------------
>  WARNING: CPU: 5 PID: 449 at drivers/pci/p2pdma.c:240 p2pmem_alloc_mmap+0x83a/0xa60
> 
> Fix by using "page_ref_count(page)" as the assertion condition.
> 
> Fixes: b7e282378773 ("mm/mm_init: move p2pdma page refcount initialisation to p2pdma")

Argh, thanks for fixing that.

Reviewed-by: Alistair Popple <apopple@...dia.com>

> Signed-off-by: Hou Tao <houtao1@...wei.com>
> ---
>  drivers/pci/p2pdma.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 218c1f5252b6..dd64ec830fdd 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -147,7 +147,7 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
>  		 * we have just allocated the page no one else should be
>  		 * using it.
>  		 */
> -		VM_WARN_ON_ONCE_PAGE(!page_ref_count(page), page);
> +		VM_WARN_ON_ONCE_PAGE(page_ref_count(page), page);
>  		set_page_count(page, 1);
>  		ret = vm_insert_page(vma, vaddr, page);
>  		if (ret) {
> -- 
> 2.29.2
>