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From:   Zi Yan <ziy@nvidia.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        David Hildenbrand <david@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Rapoport <rppt@kernel.org>,
        Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: validate buddy before check its migratetype.
Date:   Wed, 30 Mar 2022 20:15:40 -0400
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Content-Type: text/plain

On 30 Mar 2022, at 18:27, Linus Torvalds wrote:

> On Wed, Mar 30, 2022 at 3:12 PM Zi Yan <zi.yan@sent.com> wrote:
>>
>> Whenever a buddy page is found, page_is_buddy() should be called to
>> check its validity. Add the missing check during pageblock merge check.
>
> Applied.
>
>>                 buddy_pfn = __find_buddy_pfn(pfn, order);
>>                 buddy = page + (buddy_pfn - pfn);
>> +
>> +               if (!page_is_buddy(page, buddy, order))
>> +                       goto done_merging;
>
> I wonder if that sequence shouldn't be made some helper function.
>
> Also, looking around, I will note that unset_migratetype_isolate() in
> mm/page_isolation.c is missing that "page_is_buddy()" check.
>
> I _think_ it's probably ok because we checked
>
>         if (PageBuddy(page)) {
>
> on the (original, non-puddy) page, and then we only use the buddy page
> pointer for that
>
>                    if (!is_migrate_isolate_page(buddy)) {
>
> and it's been like that for a _loong_ time.
>
> But honestly, it feels like we would be better off with always having
> the page_is_buddy() check anyway.
>
> Or is there some reason why we don't want it here?
>
>              Linus

Like I said in the other email, memory isolation depends on sparsemem,
which would preclude the same NULL dereferencing from happening. But
I agree a helper function would be better. I will send a patch and see
how people think about it.


--
Best Regards,
Yan, Zi

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