From: Baolin Wang <baolin.wang@linux.alibaba.com>
To: David Hildenbrand <david@redhat.com>, akpm@linux-foundation.org
Cc: mgorman@techsingularity.net, vbabka@suse.cz, mhocko@suse.com,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page()
Date: Wed, 12 Apr 2023 20:16:27 +0800 [thread overview]
Message-ID: <ac8bb4e9-e7f5-f9da-bca0-ac7ef6d68c23@linux.alibaba.com> (raw)
In-Reply-To: <94bfa3cc-674e-25b0-e7e2-d74c970acef7@redhat.com>
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))
>
next prev parent reply other threads:[~2023-04-12 12:16 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-12 10:45 [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page() Baolin Wang
2023-04-12 11:15 ` Michal Hocko
2023-04-12 12:24 ` Baolin Wang
2023-04-12 11:25 ` David Hildenbrand
2023-04-12 12:16 ` Baolin Wang [this message]
2023-04-14 15:07 ` Vlastimil Babka
2023-04-19 6:47 ` Baolin Wang
2023-04-20 7:22 ` Huang, Ying
2023-04-20 9:11 ` Baolin Wang
2023-04-21 4:21 ` Huang, Ying
2023-04-21 7:13 ` Baolin Wang
2023-04-21 7:44 ` Huang, Ying
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