linux-kernel - Re: [PATCH] mm: page_alloc: validate buddy before check its migratetype.

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Date:   Wed, 30 Mar 2022 15:27:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Zi Yan <ziy@...dia.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Rapoport <rppt@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: validate buddy before check its migratetype.

On Wed, Mar 30, 2022 at 3:12 PM Zi Yan <zi.yan@...t.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