linux-kernel - Re: [PATCH v3] mm/page_isolation: fix potential missing call to unset_migratetype

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Date:   Wed, 15 Sep 2021 14:26:15 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     David Hildenbrand <david@...hat.com>, <akpm@...ux-foundation.org>
CC:     <mhocko@...e.com>, <vbabka@...e.cz>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] mm/page_isolation: fix potential missing call to
 unset_migratetype_isolate()

On 2021/9/15 2:13, David Hildenbrand wrote:
> On 14.09.21 13:43, Miaohe Lin wrote:
>> In start_isolate_page_range() undo path, pfn_to_online_page() just checks
>> the first pfn in a pageblock while __first_valid_page() will traverse the
>> pageblock until the first online pfn is found. So we may miss the call to
>> unset_migratetype_isolate() in undo path and pages will remain isolated
>> unexpectedly. Fix this by calling undo_isolate_page_range() and this will
>> also help to simplify the code further. Note we shouldn't ever trigger it
>> because MAX_ORDER-1 aligned pfn ranges shouldn't contain memory holes now.
>>
>> Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> 
> I read Michals reply, however, I am quite conservative with Fixes: tags. If there is nothing to fix, there is no BUG and the patch consequently merely a cleanup.
> 
> I'd have gone with a patch description/subject as follows:
> 
> "
> mm/page_isolation: cleanup start_isolate_page_range()
> 
> We can heavily simplify the code by reusing undo_isolate_page_range().
> 
> Note that this also tackles a theoretical issue that would have been a real BUG before commit c5e79ef561b0 ("mm/memory_hotplug.c: don't allow to online/offline memory blocks with holes"). In start_isolate_page_range() undo path, pfn_to_online_page() just checks
> the first pfn in a pageblock while __first_valid_page() will traverse the pageblock until the first online pfn is found. So we may miss the call to unset_migratetype_isolate() in undo path and pages will remain isolated unexpectedly.
> 
> Nowadays, start_isolate_page_range() never gets called on ranges that might contain memory holes. Consequently, this patch is not a fix but a cleanup.
> "
> 
> Anyhow, whatever the other people prefer, no strong opinion.

I have no preference too. But if this is preferred, I will do it.

> 
> Reviewed-by: David Hildenbrand <david@...hat.com>

Many thanks! :)

>