Re: [PATCH RFC 7/7] mm: better document PG_reserved

[Michal Hocko]

On Wed 05-12-18 13:28:51, David Hildenbrand wrote:
> The usage of PG_reserved and how PG_reserved pages are to be treated is
> burried deep down in different parts of the kernel. Let's shine some light
> onto these details by documenting (most?) current users and expected
> behavior.
> 
> 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.
> 
> 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>

This looks like an improvement. The essential part is that PG_reserved
page belongs to its user and no generic code should touch it. The rest
is a description of current users which I haven't checked due to to lack
of time but yeah, I like the updated wording because I have seen
multiple people confused from the swapped out part which is not true for
many many years. I have tried to dig out when it was actually the case
but failed.

So I cannot give my Ack because I didn't really do a real review but I
like this FWIW.

> ---
>  include/linux/page-flags.h | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 68b8495e2fbc..112526f5ba61 100644
> --- a/include/linux/page-flags.h
> +++ 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.
>   *
>   * 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
> -- 
> 2.17.2
> 

-- 
Michal Hocko
SUSE Labs