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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 > > --- > > 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; > } The change looks good! Acked-by: Balbir Singh