Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page()

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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 most cases, if the start is online, so is the end.

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 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))

--
Thanks,

David / dhildenb





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