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SubjectRe: [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online_mems() to get stable zone's values
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On 02.06.21 11:14, Oscar Salvador wrote:
> Currently, page_outside_zone_boundaries() takes zone's span_seqlock
> when reading zone_start_pfn and spanned_pages so those values are
> stable vs memory hotplug operations.
> move_pfn_range_to_zone() and remove_pfn_range_from_zone(), which are
> the functions that can change zone's values are serialized by
> mem_hotplug_lock by mem_hotplug_{begin,done}, so we can just use
> {get,put}_online_mems() on the readers.
>
> This will allow us to completely kill span_seqlock lock as no users
> will remain after this series.
>
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
> mm/page_alloc.c | 14 ++++++--------
> 1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index aaa1655cf682..296cb00802b4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -582,17 +582,15 @@ void set_pageblock_migratetype(struct page *page, int migratetype)
> static int page_outside_zone_boundaries(struct zone *zone, struct page *page)
> {
> int ret = 0;
> - unsigned seq;
> unsigned long pfn = page_to_pfn(page);
> unsigned long sp, start_pfn;
>
> - do {
> - seq = zone_span_seqbegin(zone);
> - start_pfn = zone->zone_start_pfn;
> - sp = zone->spanned_pages;
> - if (!zone_spans_pfn(zone, pfn))
> - ret = 1;
> - } while (zone_span_seqretry(zone, seq));
> + get_online_mems();
> + start_pfn = zone->zone_start_pfn;
> + sp = zone->spanned_pages;
> + if (!zone_spans_pfn(zone, pfn))
> + ret = 1;
> + put_online_mems();
>
> if (ret)
> pr_err("page 0x%lx outside node %d zone %s [ 0x%lx - 0x%lx ]\n",
>

It's worth noting that memory offlining might hold the memory hotplug
lock for quite some time. It's not a lightweight lock, compared to the
seqlock we have here.

I can see that page_outside_zone_boundaries() is only called from
bad_range(). bad_range() is only called under VM_BUG_ON_PAGE(). Still,
are you sure that it's even valid to block e.g., __free_one_page() and
others for eventually all eternity? And I think that we might just call
it from atomic context where we cannot even sleep.

Long story short, using get_online_mems() looks wrong.

Maybe the current lightweight reader/writer protection does serve a purpose?

--
Thanks,

David / dhildenb

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