On 15.12.18 01:12, Randy Dunlap wrote: > On 12/14/18 3:10 AM, David Hildenbrand wrote: >> The usage of PG_reserved and how PG_reserved pages are to be treated is >> buried deep down in different parts of the kernel. Let's shine some light >> onto these details by documenting current users and expected >> behavior. >> >> Especially, clarify on the "Some of them might not even exist" case. >> These are physical memory gaps that will never be dumped as they >> are not marked as IORESOURCE_SYSRAM. PG_reserved does in general not >> hinder anybody from dumping or swapping. In some cases, these pages >> will not be stored in the hibernation image. > > Hi, > Thanks for the doc update. > Comments below. > >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> >> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> >> Cc: Michal Hocko <mhocko@xxxxxxxx> >> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> >> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> >> Cc: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx> >> Cc: Miles Chen <miles.chen@xxxxxxxxxxxx> >> Cc: yi.z.zhang@xxxxxxxxxxxxxxx >> Cc: Dan Williams <dan.j.williams@xxxxxxxxx> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> >> --- >> include/linux/page-flags.h | 33 +++++++++++++++++++++++++++++++-- >> 1 file changed, 31 insertions(+), 2 deletions(-) >> >> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h >> index 808b4183e30d..9de2e941cbd5 100644 >> --- a/include/linux/page-flags.h >> +++ b/include/linux/page-flags.h >> @@ -17,8 +17,37 @@ >> /* >> * 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. > > by its owner. Indeed. > >> + * Pages marked as PG_reserved include: >> + * - Pages part of the kernel image (including vDSO) and similar (e.g. BIOS, >> + * initrd, HW tables) >> + * - Pages reserved or allocated early during boot (before the page allocator >> + * was initialized). This includes (depending on the architecture) the >> + * initial vmmap, initial page tables, crashkernel, elfcorehdr, and much > > VM map, This should actually be vmemmap (aka struct pages). > >> + * much more. Once (if ever) freed, PG_reserved is cleared and they will >> + * be given to the page allocator. >> + * - Pages falling into physical memory gaps - not IORESOURCE_SYSRAM. Trying >> + * to read/write these pages might end badly. Don't touch! >> + * - The zero page(s) >> + * - Pages not added to the page allocator when onlining a section because >> + * they were excluded via the online_page_callback() or because they are >> + * PG_hwpoison. >> + * - Pages allocated in the context of kexec/kdump (loaded kernel image, >> + * control pages, vmcoreinfo) >> + * - MMIO/DMA pages. 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). >> + * - Pages part of an offline section (struct pages of offline sections should >> + * not be trusted as they will be initialized when first onlined). >> + * - MCA pages on ia64 >> + * - Pages holding CPU notes for POWER Firmware Assisted Dump >> + * - Device memory (e.g. PMEM, DAX, HMM) >> + * Some PG_reserved pages will be excluded from the hibernation image. >> + * PG_reserved does in general not hinder anybody from dumping or swapping >> + * and is no longer required for remap_pfn_range(). ioremap might require it. >> + * Consequently, PG_reserved for a page mapped into user space can indicate >> + * the zero page, the vDSO, MMIO pages or device memory. >> * >> * The PG_private bitflag is set on pagecache pages if they contain filesystem >> * specific data (which is normally at page->private). It can be used by >> > > cheers. > Thanks! -- Thanks, David / dhildenb
