Re: [PATCH RFC 2/8] mm: introduce PG_offline

[David Hildenbrand <david@redhat.com>]

On 13.04.2018 15:16, David Hildenbrand wrote:
> online_pages()/offline_pages() theoretically allows us to work on
> sub-section sizes. This is especially relevant in the context of
> virtualization. It e.g. allows us to add/remove memory to Linux in a VM in
> 4MB chunks.
> 
> While the whole section is marked as online/offline, we have to know
> the state of each page. E.g. to not read memory that is not online
> during kexec() or to properly mark a section as offline as soon as all
> contained pages are offline.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/linux/page-flags.h     | 10 ++++++++++
>  include/trace/events/mmflags.h |  9 ++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index e34a27727b9a..8ebc4bad7824 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -49,6 +49,9 @@
>   * PG_hwpoison indicates that a page got corrupted in hardware and contains
>   * data with incorrect ECC bits that triggered a machine check. Accessing is
>   * not safe since it may cause another machine check. Don't touch!
> + *
> + * PG_offline indicates that a page is offline and the backing storage
> + * might already have been removed (virtualization). Don't touch!
>   */
>  
>  /*
> @@ -100,6 +103,9 @@ enum pageflags {
>  #if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
>  	PG_young,
>  	PG_idle,
> +#endif
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +	PG_offline,		/* Page is offline. Don't touch */
>  #endif
>  	__NR_PAGEFLAGS,
>  
> @@ -381,6 +387,10 @@ TESTCLEARFLAG(Young, young, PF_ANY)
>  PAGEFLAG(Idle, idle, PF_ANY)
>  #endif
>  
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +PAGEFLAG(Offline, offline, PF_ANY)
> +#endif
> +
>  /*
>   * On an anonymous page mapped into a user virtual memory area,
>   * page->mapping points to its anon_vma, not to a struct address_space;
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index a81cffb76d89..14c31209e34a 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -79,6 +79,12 @@
>  #define IF_HAVE_PG_IDLE(flag,string)
>  #endif
>  
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +#define IF_HAVE_PG_OFFLINE(flag,string) ,{1UL << flag, string}
> +#else
> +#define IF_HAVE_PG_OFFLINE(flag,string)
> +#endif
> +
>  #define __def_pageflag_names						\
>  	{1UL << PG_locked,		"locked"	},		\
>  	{1UL << PG_waiters,		"waiters"	},		\
> @@ -104,7 +110,8 @@ IF_HAVE_PG_MLOCK(PG_mlocked,		"mlocked"	)		\
>  IF_HAVE_PG_UNCACHED(PG_uncached,	"uncached"	)		\
>  IF_HAVE_PG_HWPOISON(PG_hwpoison,	"hwpoison"	)		\
>  IF_HAVE_PG_IDLE(PG_young,		"young"		)		\
> -IF_HAVE_PG_IDLE(PG_idle,		"idle"		)
> +IF_HAVE_PG_IDLE(PG_idle,		"idle"		)		\
> +IF_HAVE_PG_OFFLINE(PG_offline,		"offline"	)
>  
>  #define show_page_flags(flags)						\
>  	(flags) ? __print_flags(flags, "|",				\
> 

I am thinking right now of gluing this to CONFIG_MEMORY_PG_OFFLINE and
allowing it to be used also for ordinary balloon drivers. It is then
basically a way to signal using kdump "don't dump this page, the content
is either invalid or not even accessible".

-- 

Thanks,

David / dhildenb