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

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Message-ID: <20260112005440.998543-1-apopple@nvidia.com>
Date: Mon, 12 Jan 2026 11:54:40 +1100
From: Alistair Popple <apopple@...dia.com>
To: helgaas@...nel.org
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,
	Alistair Popple <apopple@...dia.com>
Subject: [PATCH] PCI/P2PDMA: Reset page reference count when page mapping fails

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>

---

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