linux-kernel - Re: [PATCH 2/2] mm,memory_hotplug: {READ,WRITE}

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Date: Wed, 22 May 2024 16:05:12 +0200
From: David Hildenbrand <david@...hat.com>
To: Brendan Jackman <jackmanb@...gle.com>, Oscar Salvador
 <osalvador@...e.de>, Andrew Morton <akpm@...ux-foundation.org>,
 Mike Rapoport <rppt@...nel.org>
Cc: Michal Hocko <mhocko@...e.com>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Vlastimil Babka <vbabka@...e.cz>, Pavel Tatashin
 <pasha.tatashin@...een.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm,memory_hotplug: {READ,WRITE}_ONCE unsynchronized
 zone data

On 21.05.24 14:57, Brendan Jackman wrote:
> These fields are written by memory hotplug under mem_hotplug_lock but
> read without any lock. It seems like reader code is robust against the
> value being stale or "from the future", but we also need to account
> for:
> 
> 1. Load/store tearing (according to Linus[1], this really happens,
>     even when everything is aligned as you would hope).
> 
> 2. Invented loads[2] - the compiler can spill and re-read these fields
>     ([2] calls this "invented loads") and assume that they have not
>     changed.
> 
> Note we don't need READ_ONCE in paths that have the mem_hotplug_lock
> for write, but we still need WRITE_ONCE to prevent store-tearing.
> 
> [1] https://lore.kernel.org/all/CAHk-=wj2t+GK+DGQ7Xy6U7zMf72e7Jkxn4_-kGyfH3WFEoH+YQ@mail.gmail.com/T/#u
>      As discovered via the original big-bad article[2]
> [2] https://lwn.net/Articles/793253/
> 
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
>   include/linux/mmzone.h | 14 ++++++++++----
>   mm/compaction.c        |  2 +-
>   mm/memory_hotplug.c    | 20 ++++++++++++--------
>   mm/mm_init.c           |  2 +-
>   mm/page_alloc.c        |  2 +-
>   mm/show_mem.c          |  8 ++++----
>   mm/vmstat.c            |  4 ++--
>   7 files changed, 31 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 194ef7fed9d6..bdb3be76d10c 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1018,11 +1018,13 @@ static inline unsigned long zone_cma_pages(struct zone *zone)
>   #endif
>   }
>   
> +/* This is unstable unless you hold mem_hotplug_lock. */
>   static inline unsigned long zone_end_pfn(const struct zone *zone)
>   {
> -	return zone->zone_start_pfn + zone->spanned_pages;
> +	return zone->zone_start_pfn + READ_ONCE(zone->spanned_pages);

It's weird to apply that logic only to spanned_pages, whereby 
zone_start_pfn can (and will) similarly change when onlining/offlining 
memory.

-- 
Cheers,

David / dhildenb