On Wed, Dec 05, 2018 at 01:28:51PM +0100, David Hildenbrand wrote:
I don't see a reason why we have to document "Some of them might not even exist". If there is a user, we should document it. E.g. for balloon drivers we now use PG_offline to indicate that a page might currently not be backed by memory in the hypervisor. And that is independent from PG_reserved.
I think you're confused by the meaning of "some of them might not even exist". What this means is that there might not be memory there; maybe writes to that memory will be discarded, or maybe they'll cause a machine check. Maybe reads will return ~0, or 0, or cause a machine check. We just don't know what's there, and we shouldn't try touching the memory.
+++ b/include/linux/page-flags.h @@ -17,8 +17,22 @@ /* * Various page->flags bits: * - * PG_reserved is set for special pages, which can never be swapped out. Some - * of them might not even exist... + * PG_reserved is set for special pages. The "struct page" of such a page + * should in general not be touched (e.g. set dirty) except by their owner. + * Pages marked as PG_reserved include: + * - Kernel image (including vDSO) and similar (e.g. BIOS, initrd) + * - Pages allocated early during boot (bootmem, memblock) + * - Zero pages + * - Pages that have been associated with a zone but are not available for + * the page allocator (e.g. excluded via online_page_callback()) + * - Pages to exclude from the hibernation image (e.g. loaded kexec images) + * - MMIO pages (communicate with a device, special caching strategy needed) + * - MCA pages on ia64 (pages with memory errors) + * - Device memory (e.g. PMEM, DAX, HMM) + * Some architectures don't allow to ioremap pages that are not marked + * PG_reserved (as they might be in use by somebody else who does not respect + * the caching strategy). Consequently, PG_reserved for a page mapped into + * user space can indicate the zero page, the vDSO, MMIO pages or device memory.
So maybe just add one more option to the list.
