linux-kernel - Re: [PATCH] PCI/P2PDMA: Reset page reference count when page mapping fails

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Message-ID: <20260112171316.GA716207@bhelgaas>
Date: Mon, 12 Jan 2026 11:13:16 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Alistair Popple <apopple@...dia.com>
Cc: houtao@...weicloud.com, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-mm@...ck.org,
	linux-nvme@...ts.infradead.org, bhelgaas@...gle.com,
	logang@...tatee.com, leonro@...dia.com, gregkh@...uxfoundation.org,
	tj@...nel.org, rafael@...nel.org, dakr@...nel.org,
	akpm@...ux-foundation.org, david@...nel.org,
	lorenzo.stoakes@...cle.com, kbusch@...nel.org, axboe@...nel.dk,
	hch@....de, sagi@...mberg.me, houtao1@...wei.com
Subject: Re: [PATCH] PCI/P2PDMA: Reset page reference count when page mapping
 fails

On Mon, Jan 12, 2026 at 11:54:40AM +1100, Alistair Popple wrote:
> When mapping a p2pdma page the page reference count is initialised to
> 1 prior to calling vm_insert_page(). This is to avoid vm_insert_page()
> warning if the page refcount is zero. Prior to setting the page count
> there is a check to ensure the page is currently free (ie. has a zero
> reference count).
> 
> However vm_insert_page() can fail. In this case the pages are freed
> back to the genalloc pool, but that does not reset the page refcount.
> So a future allocation of the same page will see the elevated page
> refcount from the previous set_page_count() call triggering the
> VM_WARN_ON_ONCE_PAGE checking that the page is free.
> 
> Fix this by resetting the page refcount back to zero using
> set_page_count(). Note that put_page() is not used because that
> would result in freeing the page twice due to implicitly calling
> p2pdma_folio_free().
> 
> Fixes: b7e282378773 ("mm/mm_init: move p2pdma page refcount initialisation to p2pdma")
> Signed-off-by: Alistair Popple <apopple@...dia.com>

Applied to pci/p2pdma for v6.20, thanks!

> ---
> 
> This was found by inspection. I don't currently have a good setup that
> exercises the p2pmem_alloc_mmap() path so this has only been compile
> tested - additional testing would be appreciated.
> ---
>  drivers/pci/p2pdma.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index dd64ec830fdd..3b29246b9e86 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -152,6 +152,12 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
>  		ret = vm_insert_page(vma, vaddr, page);
>  		if (ret) {
>  			gen_pool_free(p2pdma->pool, (uintptr_t)kaddr, len);
> +
> +			/*
> +			 * Reset the page count. We don't use put_page() because
> +			 * we don't want to trigger the p2pdma_folio_free() path.
> +			 */
> +			set_page_count(page, 0);
>  			percpu_ref_put(ref);
>  			return ret;
>  		}
> -- 
> 2.51.0
>