Re: [PATCH v1 9/9] mm: better document PG_reserved

[Randy Dunlap]

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.

> + * 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,

> + *   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.
-- 
~Randy