From: David Hildenbrand <david@redhat.com>
To: Oscar Salvador <osalvador@suse.de>,
Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
Michal Hocko <mhocko@kernel.org>,
Anshuman Khandual <anshuman.khandual@arm.com>,
Vlastimil Babka <vbabka@suse.cz>,
Pavel Tatashin <pasha.tatashin@soleen.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online_mems() to get stable zone's values
Date: Wed, 2 Jun 2021 20:37:02 +0200 [thread overview]
Message-ID: <39473305-6e91-262d-bcc2-76b745a5b14a@redhat.com> (raw)
In-Reply-To: <20210602091457.17772-2-osalvador@suse.de>
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
next prev parent reply other threads:[~2021-06-02 18:37 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-02 9:14 [PATCH v2 0/3] Memory hotplug locking cleanup Oscar Salvador
2021-06-02 9:14 ` [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online_mems() to get stable zone's values Oscar Salvador
2021-06-02 18:37 ` David Hildenbrand [this message]
2021-06-02 19:45 ` Oscar Salvador
2021-06-03 8:38 ` Oscar Salvador
2021-06-03 12:45 ` Michal Hocko
2021-06-04 7:41 ` Oscar Salvador
2021-06-07 7:52 ` Oscar Salvador
2021-06-07 8:49 ` David Hildenbrand
2021-06-07 10:23 ` Oscar Salvador
2021-06-08 10:42 ` Oscar Salvador
2021-06-08 15:00 ` David Hildenbrand
2021-06-09 9:42 ` David Hildenbrand
2021-06-07 8:42 ` Michal Hocko
2021-06-03 2:32 ` [mm,page_alloc] [confidence: ] acb5758bf4: BUG:sleeping_function_called_from_invalid_context_at_include/linux/percpu-rwsem.h kernel test robot
2021-06-02 9:14 ` [PATCH v2 2/3] mm,memory_hotplug: Drop unneeded locking Oscar Salvador
2021-06-03 12:52 ` Michal Hocko
2021-06-02 9:14 ` [PATCH v2 3/3] mm,memory_hotplug: Remove unneeded declarations Oscar Salvador
2021-06-02 18:38 ` David Hildenbrand
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