On Mon, Aug 19, 2019 at 2:32 AM Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx> wrote: > > Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx> writes: > > > Dan Williams <dan.j.williams@xxxxxxxxx> writes: > > > >> On Fri, Aug 9, 2019 at 12:45 AM Aneesh Kumar K.V > >> <aneesh.kumar@xxxxxxxxxxxxx> wrote: > >>> > >> > > ... > > >>> diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c > >>> index 37e96811c2fc..c1d9be609322 100644 > >>> --- a/drivers/nvdimm/pfn_devs.c > >>> +++ b/drivers/nvdimm/pfn_devs.c > >>> @@ -725,7 +725,8 @@ static int nd_pfn_init(struct nd_pfn *nd_pfn) > >>> * when populating the vmemmap. This *should* be equal to > >>> * PMD_SIZE for most architectures. > >>> */ > >>> - offset = ALIGN(start + SZ_8K + 64 * npfns, align) - start; > >>> + offset = ALIGN(start + SZ_8K + sizeof(struct page) * npfns, > >> > >> I'd prefer if this was not dynamic and was instead set to the maximum > >> size of 'struct page' across all archs just to enhance cross-arch > >> compatibility. I think that answer is '64'. > > > > > > That still doesn't take care of the case where we add new elements to > > struct page later. If we have struct page size changing across > > architectures, we should still be ok as long as new size is less than what is > > stored in pfn superblock? I understand the desire to keep it > > non-dynamic. But we also need to make sure we don't reserve less space > > when creating a new namespace on a config that got struct page size > > > 64? > > > How about > > libnvdimm/pfn_dev: Add a build check to make sure we notice when struct page size change > > When namespace is created with map device as pmem device, struct page is stored in the > reserve block area. We need to make sure we account for the right struct page > size while doing this. Instead of directly depending on sizeof(struct page) > which can change based on different kernel config option, use the max struct > page size (64) while calculating the reserve block area. This makes sure pmem > device can be used across kernels built with different configs. > > If the above assumption of max struct page size change, we need to update the > reserve block allocation space for new namespaces created. > > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx> > > 1 file changed, 7 insertions(+) > drivers/nvdimm/pfn_devs.c | 7 +++++++ > > modified drivers/nvdimm/pfn_devs.c > @@ -722,7 +722,14 @@ static int nd_pfn_init(struct nd_pfn *nd_pfn) > * The altmap should be padded out to the block size used > * when populating the vmemmap. This *should* be equal to > * PMD_SIZE for most architectures. > + * > + * Also make sure size of struct page is less than 64. We > + * want to make sure we use large enough size here so that > + * we don't have a dynamic reserve space depending on > + * struct page size. But we also want to make sure we notice > + * if we end up adding new elements to struct page. > */ > + BUILD_BUG_ON(64 < sizeof(struct page)); Looks ok to me. There are ongoing heroic efforts to make sure 'struct page' does not grown beyond the size of cacheline. The fact that 'struct page_ext' is allocated out of line makes it safe to assume that 'struct page' will not be growing larger in the foreseeable future.
