On 4/12/2023 7:25 PM, David Hildenbrand wrote:
On 12.04.23 12:45, Baolin Wang wrote:
Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
which checks whether the given zone contains holes, and uses pfn_valid()
to check if the end pfn is valid. However pfn_valid() can not make sure
the end pfn is not a hole if the size of a pageblock is larger than the
size of a sub-mem_section, since the struct page getting by pfn_to_page()
may represent a hole or an unusable page frame, which may cause incorrect
zone contiguous is set.
Though another user of pageblock_pfn_to_page() in compaction seems work
well now, it is better to avoid scanning or touching these offline pfns.
So like commit 2d070eab2e82 ("mm: consider zone which is not fully
populated to have holes"), we should also use pfn_to_online_page() for
the end pfn to make sure it is a valid pfn with usable page frame.
Meanwhile the pfn_valid() for end pfn can be dropped now.
Moreover we've already used pfn_to_online_page() for start pfn to make
sure it is online and valid, so the pfn_valid() for the start pfn is
unnecessary, drop it.
pageblocks are supposed to fall into a single memory section, so in mos > cases, if the start is online, so is the end.
Yes, the granularity of memory hotplug is a mem_section.
However, suppose the pageblock order is MAX_ORDER-1, and the size of a
sub-section is 2M, that means a pageblock will fall into 2 sub
mem-section, and if there is a hole in the zone, that means the 2nd sub
mem-section can be invalid without setting subsection_map bitmap.
So the start is online can make sure the end pfn of a pageblock is
online, but a valid start pfn can not make sure the end pfn is valid in
the bitmap of ms->usage->subsection_map.
The exception to this rule is when we have a mixture of ZONE_DEVICE and
ZONE_* within the same section.
Then, indeed the end might not be online.
BUT, if the end is valid (-> ZONE_DEVICE), then the zone_id will differ.
[let's ignore any races for now, up to this point they are mostly of
theoretical nature]
So I don't think this change actually fixes something.
Getting rid of the pfn_valid(start_pfn) makes sense. Replacing the
Yes, my motivation is try to optimize the __pageblock_pfn_to_page()
which is hot when doing compaction, and I saw these pfn_valid() can be
dropped.
pfn_valid(end_pfn) by a pfn_to_online_page(end_pfn) could make that
function less efficient.
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d0eb280ec7e4..8076f519c572 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1512,9 +1512,6 @@ struct page *__pageblock_pfn_to_page(unsigned
long start_pfn,
/* end_pfn is one past the range we are checking */
end_pfn--;
- if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
- return NULL;
-
start_page = pfn_to_online_page(start_pfn);
if (!start_page)
return NULL;
@@ -1522,7 +1519,9 @@ struct page *__pageblock_pfn_to_page(unsigned
long start_pfn,
if (page_zone(start_page) != zone)
return NULL;
- end_page = pfn_to_page(end_pfn);
+ end_page = pfn_to_online_page(end_pfn);
+ if (!end_page)
+ return NULL;
/* This gives a shorter code than deriving page_zone(end_page) */
if (page_zone_id(start_page) != page_zone_id(end_page))
