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

[Logan Gunthorpe <logang@deltatee.com>]

Hi Alistair,

On 2026-01-11 17:54, 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@nvidia.com>

I had time today to pull out some old hardware and run this patch
through my old p2pdma smoke tests. I don't think the tests exercise the
change, but it looks correct to me on review.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

Thanks for the fix!

Logan