linux-kernel - Re: [PATCH v2 1/4] mm: Introduce vm_uffd

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Message-ID: <aMnDAdMyuatKth3w@x1.local>
Date: Tue, 16 Sep 2025 16:05:21 -0400
From: Peter Xu <peterx@...hat.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
	Muchun Song <muchun.song@...ux.dev>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Houghton <jthoughton@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Oscar Salvador <osalvador@...e.de>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Ujwal Kundur <ujwal.kundur@...il.com>
Subject: Re: [PATCH v2 1/4] mm: Introduce vm_uffd_ops API

On Mon, Sep 08, 2025 at 12:53:37PM -0400, Liam R. Howlett wrote:
> What we don't want is non-mm code specifying values for the function
> pointer and doing what they want, or a function pointer that returns a
> core mm resource (in the old example this was a vma, here it is a
> folio).
> 
> From this patch set:
> +        * Return: zero if succeeded, negative for errors.
> +        */
> +       int (*uffd_get_folio)(struct inode *inode, pgoff_t pgoff,
> +                             struct folio **folio);
> 
> This is one of the contention points in the current scenario as the
> folio would be returned.

OK I didn't see this one previously, it partly answers one of my question
in the other reply, in a way I wished not.

Could you elaborate why an API returning an folio pointer would be
dangerous?

OTOH, would you think alloc_pages() or folio_alloc() be dangerous too?

They return a folio from the mm core to drivers, hence it's not the same
direction of folio sharing, however it also means at least the driver can
manipulate the folio / memmap as much as it wants, sabotaging everything is
similarly possible.  Why we worry about that?

Are we going to unexport alloc_pages() someday?

Thanks,

-- 
Peter Xu