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Date:   Wed, 12 Apr 2023 13:25:32 +0200
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To:     Baolin Wang <baolin.wang@linux.alibaba.com>,
        akpm@linux-foundation.org
Cc:     mgorman@techsingularity.net, vbabka@suse.cz, mhocko@suse.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <62e231a8f2e50c04dcadc7a0cfaa6dea5ce1ec05.1681296022.git.baolin.wang@linux.alibaba.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in
 __pageblock_pfn_to_page()
In-Reply-To: <62e231a8f2e50c04dcadc7a0cfaa6dea5ce1ec05.1681296022.git.baolin.wang@linux.alibaba.com>
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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@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))

-- 
Thanks,

David / dhildenb