On Tue, Feb 5, 2019 at 6:05 PM Oliver O'Halloran <oohall@xxxxxxxxx> wrote:
>
> Libnvdimm reserves the first 8K of pfn and devicedax namespaces to
> store a superblock describing the namespace. This 8K reservation
> is contained within the altmap area which the kernel uses for the
> vmemmap backing for the pages within the namespace. The altmap
> allows for some pages at the start of the altmap area to be reserved
> and that mechanism is used to protect the superblock from being
> re-used as vmemmap backing.
>
> The number of PFNs to reserve is calculated using:
>
> PHYS_PFN(SZ_8K)
>
> Which is implemented as:
>
> #define PHYS_PFN(x) ((unsigned long)((x) >> PAGE_SHIFT))
>
> So on systems where PAGE_SIZE is greater than 8K the reservation
> size is truncated to zero and the superblock area is re-used as
> vmemmap backing. As a result all the namespace information stored
> in the superblock (i.e. if it's a PFN or DAX namespace) is lost
> and the namespace needs to be re-created to get access to the
> contents.
>
> This patch fixes this by using PFN_UP() rather than PHYS_PFN() to ensure
> that at least one page is reserved. On systems with a 4K pages size this
> patch should have no effect.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Fixes: ac515c084be9 ("libnvdimm, pmem, pfn: move pfn setup to the core")
> Signed-off-by: Oliver O'Halloran <oohall@xxxxxxxxx>
> ---
> ---
> drivers/nvdimm/pfn_devs.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c
> index 6f22272e8d80..9b9be83da0e7 100644
> --- a/drivers/nvdimm/pfn_devs.c
> +++ b/drivers/nvdimm/pfn_devs.c
> @@ -593,7 +593,7 @@ static unsigned long init_altmap_base(resource_size_t base)
>
> static unsigned long init_altmap_reserve(resource_size_t base)
> {
> - unsigned long reserve = PHYS_PFN(SZ_8K);
> + unsigned long reserve = PFN_UP(SZ_8K);
> unsigned long base_pfn = PHYS_PFN(base);
Good find. I know how annoying it is to debug altmap issues.
I'll get this queued up.
