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Date:   Mon, 15 Aug 2022 16:26:46 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Charan Teja Kalla <quic_charante@quicinc.com>,
        akpm@linux-foundation.org, david@redhat.com,
        pasha.tatashin@soleen.com, sieberf@amazon.com, shakeelb@google.com,
        sjpark@amazon.de, dhowells@redhat.com, quic_pkondeti@quicinc.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH V3] mm: fix use-after free of page_ext after race with
 memory-offline
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 <Yvpg6odyDsXrjw5i@dhcp22.suse.cz>
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On Mon, Aug 15, 2022 at 05:06:18PM +0200, Michal Hocko wrote:
> > + * This function gives proper page_ext of a memory section
> > + * during race with the offline operation on a memory block
> > + * this section falls into. Not using this function to get
> > + * page_ext of a page, in code paths where extra refcount
> > + * is not taken on that page eg: pfn walking, can lead to
> > + * use-after-free access of page_ext.
> 
> I do not think this is really useful comment, it goes into way too much
> detail about memory hotplug yet not enough to actually understand the
> interaction because there are no references to the actual
> synchronization scheme. I would go with something like:
> 
> /*
>  * Get a page_ext associated with the given page. Returns NULL if
>  * no such page_ext exists otherwise ensures that the page_ext will
>  * stay alive until page_ext_put is called.
>  * This implies a non-sleeping context.
>  */

I'd go further and turn this into kernel-doc:

/**
 * page_ext_get() - Get the extended information for a page.
 * @page: The page we're interested in.
 *
 * Ensures that the page_ext will remain valid until page_ext_put()
 * is called.
 * 
 * Return: NULL if no page_ext exists for this page.
 * Context: Any context.  Caller may not sleep until they have called
 * page_ext_put().
 */