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

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

[Alistair Popple]

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@nvidia.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