Messages in this thread | | | Date | Wed, 12 Apr 2023 20:16:27 +0800 | Subject | Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page() | From | Baolin Wang <> |
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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@linux.alibaba.com> >> --- >> 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)) >
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