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

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Message-ID: <du5pzxmfk6lile3ykpaloylwz4eni6disj2oe25eq6ipzqemiw@ybcouflfnlev>
Date: Wed, 17 Sep 2025 12:53:05 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Nikita Kalyazin <kalyazin@...zon.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>,
        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

* Mike Rapoport <rppt@...nel.org> [250917 05:26]:
> Hi Liam,
> 
> On Mon, Sep 08, 2025 at 12:53:37PM -0400, Liam R. Howlett wrote:
> > 
> > Reading through the patches, I'm not entirely sure what you are
> > proposing.
> > 
> > What I was hoping to see by a generalization of the memory types is a
> > much simpler shared code base until the code hit memory type specific
> > areas where a function pointer could be used to keep things from getting
> > complicated (or, I guess a switch statement..).
> > 
> > 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.
> 
> I don't see a problem with it. It's not any different from
> vma_ops->fault(): a callback for a filesystem to get a folio that will be
> mapped afterwards by the mm code.
> 

I disagree, the filesystem vma_ops->fault() is not a config option like
this one.  So we are on a path to enable uffd by default, and it really
needs work beyond this series.  Setting up a list head and passing in
through every call stack is far from idea.

I also think the filesystem model is not one we want to duplicate in mm
for memory types - think of the test issues we have now and then have a
look at the xfstests support of filesystems [1].

So we are on a path of less test coverage, and more code that is
actually about mm that is outside of mm.  So, is there another way?

Cheers,
Liam

[1]. https://github.com/kdave/xfstests/blob/master/README#L37