linux-kernel - Re: [PATCH v2] mm/page_isolation: fix potential missing call to unset_migratetype

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Message-ID: <1d0e15ab-4a1f-0820-26c2-4b9949164dfa@huawei.com>
Date:   Tue, 14 Sep 2021 11:09:47 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     David Hildenbrand <david@...hat.com>, <vbabka@...e.cz>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm/page_isolation: fix potential missing call to
 unset_migratetype_isolate()

On 2021/9/13 20:59, Michal Hocko wrote:
> On Mon 13-09-21 20:43:35, Miaohe Lin wrote:
>> On 2021/9/13 20:20, David Hildenbrand wrote:
>>> On 13.09.21 14:12, Michal Hocko wrote:
>>>> On Mon 13-09-21 19:51:25, 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.
>>>>
>>>> I like the clean up part but is this a real problem that requires CC
>>>> stable? Have you ever seen this to be a real problem? It looks like
>>>> something based on reading the code.
>>
>> I'm sorry but I haven't seen this to be a real problem. It's a theoretical bug.
> 
> Make it clear in the changelog

Will do.

> 
>>> We discussed that it isn't an issue anymore (we never call it on
>>> memory holes), but might have been an issue on older kernels, back
>>> when we didn't have the "memory holes" check in the memory offlining
>>> path in place.
>>
>> So is the Cc:stable needed in this case?
> 
> I do not think so. Even if this was happening in the practice then the
> practical consequences would be pretty minor, right? (few pageblocks
> stay isolated thus unavailable).
> 
> I do realize that the stable tree is in a hoarding mode for quite some
> years but my general approach has been (in line with the documentation)
> to mark and backport only fixes that really do matter.

So even the Fixes tag should be removed ?

Many thanks.

>