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@xxxxxxx>
---
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
