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

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Message-ID: <YT9LPoSSTJbmF3eF@dhcp22.suse.cz>
Date:   Mon, 13 Sep 2021 14:59:42 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        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 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

> > 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.
-- 
Michal Hocko
SUSE Labs