linux-kernel - Re: [PATCH v2] mm/hotplug: fix offline undo_isolate_page

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Date:   Thu, 14 Mar 2019 09:09:22 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, mhocko@...nel.org, vbabka@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/hotplug: fix offline undo_isolate_page_range()

On Wed, Mar 13, 2019 at 10:31:33AM -0400, Qian Cai wrote:
> Also, after calling the "useless" undo_isolate_page_range() here, it
> reaches the point of no returning by notifying MEM_OFFLINE. Those pages
> will be marked as MIGRATE_MOVABLE again once onlining. The only thing
> left to do is to decrease the number of isolated pageblocks zone
> counter which would make some paths of the page allocation slower that
> the above commit introduced. A memory block is usually at most 1GiB in
> size, so an "int" should be enough to represent the number of pageblocks
> in a block. Fix an incorrect comment along the way.

Well, x86_64's memblocks can be up to 2GB depending on the memory we got.
Plus the fact that alloc_contig_range can be used to isolate 16GB-hugetlb
pages on powerpc, and that could be a lot of pageblocks.

While an "int" could still hold, I think we should use "long" just to be
more future-proof.

> 
> Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")
> Signed-off-by: Qian Cai <cai@....pw>
> ---
> 
> v2: return the nubmer of isolated pageblocks in start_isolate_page_range() per
>     Oscar; take the zone lock when undoing zone->nr_isolate_pageblock per
>     Michal.
> 
>  mm/memory_hotplug.c | 17 +++++++++++++----
>  mm/page_alloc.c     |  2 +-
>  mm/page_isolation.c | 16 ++++++++++------
>  mm/sparse.c         |  2 +-
>  4 files changed, 25 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index cd23c081924d..8ffe844766da 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1580,7 +1580,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  {
>  	unsigned long pfn, nr_pages;
>  	long offlined_pages;
> -	int ret, node;
> +	int ret, node, count;

I would rather use a meaningful name here, like "nr_isolated_pageblocks" or
something like that.
 

-- 
Oscar Salvador
SUSE L3