Dan Williams <dan.j.willi...@intel.com> writes: > On Thu, Feb 13, 2020 at 8:58 AM Jeff Moyer <jmo...@redhat.com> wrote:
>> I have just a couple of questions. >> >> First, can you please add a comment above the generic implementation of >> memremap_compat_align describing its purpose, and why a platform might >> want to override it? > > Sure, how about: > > /* > * The memremap() and memremap_pages() interfaces are alternately used > * to map persistent memory namespaces. These interfaces place different > * constraints on the alignment and size of the mapping (namespace). > * memremap() can map individual PAGE_SIZE pages. memremap_pages() can > * only map subsections (2MB), and at least one architecture (PowerPC) > * the minimum mapping granularity of memremap_pages() is 16MB. > * > * The role of memremap_compat_align() is to communicate the minimum > * arch supported alignment of a namespace such that it can freely > * switch modes without violating the arch constraint. Namely, do not > * allow a namespace to be PAGE_SIZE aligned since that namespace may be > * reconfigured into a mode that requires SUBSECTION_SIZE alignment. > */ Well, if we modify the x86 variant to be PAGE_SIZE, I think that text won't work. How about: /* * memremap_compat_align should return the minimum alignment for * mapping memory via memremap() and memremap_pages(). For x86, this * is the system PAGE_SIZE. Other architectures may impose different * restrictions, as is seen on powerpc where the minimum alignment is * tied to the linear mapping page size. * * When creating persistent memory namespaces, the alignment is forced * to the least common denominator (MEMREMAP_COMPAT_ALIGN_MAX, * currently 16MB). However, older kernels did not enforce this * behavior, so we allow mapping namespaces with smaller alignments, * so long as the platform supports it. See nvdimm_namespace_common_probe. */ -Jeff
