linux-kernel - Re: [PATCH v4 0/4] Make alloc_contig

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Message-ID: <d2ab8528-e2c5-b876-7ca0-03faa86fba25@redhat.com>
Date:   Wed, 17 Mar 2021 11:07:00 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v4 0/4] Make alloc_contig_range handle Hugetlb pages

On 17.03.21 11:05, Oscar Salvador wrote:
> On Wed, Mar 17, 2021 at 10:48:31AM +0100, David Hildenbrand wrote:
>>> I was preparing v5, and I wanted to be sure I understood you here.
>>>
>>> Right you are that the in-use page check can be dropped, as those pages
>>> can
>>> be migrated away, and the Hugetlb page check can also be dropped since
>>> isolate_migratepages_range is now capable of dealing with those kind of
>>> pages.
>>>
>>>> b) Similarly, check for gigantic pages and/or movability/migratability.
>>>
>>> I lost you here.
>>>
>>> isolate_or_dissolve_huge_page() already bails out on hugetlb-gigantic
>>> pages.
>>>
>>> Or do you mean to place an upfront check here? (hstate_is_gigantic())?
>>
>> Yes. But I prefer a) and keeping it simple here -- just doing basic sanity
>> checks (online, zone, PageReserved()) that are absolutely necessary.
> 
> Ok, I am probably dense as I understood as if you were lean towards having
> a) + b).

Sorry, I meant either a) or b) :)

> 
> That is what I have as the last patch of the patchset:
> 
>  From e97175b7d4970cbdcbafcf8c398f72a571e817b0 Mon Sep 17 00:00:00 2001
> From: Oscar Salvador <osalvador@...e.de>
> Date: Thu, 18 Mar 2021 05:03:18 +0100
> Subject: [PATCH] mm,page_alloc: Drop unnecesary checks from
>   pfn_range_valid_contig
> 
> pfn_range_valid_contig() bails out when it finds an in-use page or a
> hugetlb page, among other things.
> We can drop the in-use page check since __alloc_contig_pages can migrate
> away those pages, and the hugetlb page check can go too since
> isolate_migratepages_range is now capable of dealing with hugetlb pages.
> 

Might want to mention that the existing checks were racy either way :)

> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>   mm/page_alloc.c | 6 ------
>   1 file changed, 6 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4cb455355f6d..50d73e68b79e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8685,12 +8685,6 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
> 
>                  if (PageReserved(page))
>                          return false;
> -
> -               if (page_count(page) > 0)
> -                       return false;
> -
> -               if (PageHuge(page))
> -                       return false;
>          }
>          return true;
>   }
> 


-- 
Thanks,

David / dhildenb