linux-kernel - Re: [PATCH v3] mm/page_isolation: guard against possible putback unisolated page

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Date:   Wed, 15 Sep 2021 14:37:46 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     <david@...hat.com>, <jhubbard@...dia.com>, <vbabka@...e.cz>,
        <iamjoonsoo.kim@....com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] mm/page_isolation: guard against possible putback
 unisolated page

On 2021/9/15 10:09, Andrew Morton wrote:
> On Tue, 14 Sep 2021 19:45:08 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:
> 
>> Isolating a free page in an isolated pageblock is expected to always work
>> as watermarks don't apply here. But if __isolate_free_page() failed, due
>> to condition changes, the page will be left on the free list. And the page
>> will be put back to free list again via __putback_isolated_page(). This may
>> trigger VM_BUG_ON_PAGE() on page->flags checking in __free_one_page() if
>> PageReported is set. Or we will corrupt the free list because list_add()
>> will be called for pages already on another list. Add a VM_WARN_ON() to
>> complain about this change.
> 
> Are you able to identify a Fixes: here?
> 

Sure, this should be "Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated pageblock")."

> Is a cc:stable justified?

I'm afraid not. As David pointed out,
""

In unset_migratetype_isolate() we check that is_migrate_isolate_page(page) holds, otherwise we return.

We call __isolate_free_page() only for such pages.

__isolate_free_page() won't perform watermark checks on is_migrate_isolate().

Consequently, __isolate_free_page() should never fail when called from unset_migratetype_isolate()


""

In a nutshell, __isolate_free_page can not fail here. So it is harmless now and doesn't worth cc:stable.

Please see https://www.spinics.net/lists/linux-mm/msg269434.html for detail.

Many thanks.

> .
>