linux-kernel - Re: [RFC PATCH 1/3] mm: introduce new .mmap_proto() f

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Message-ID: <e505a8d2-2407-441d-8225-b7f94bc2b953@lucifer.local>
Date: Thu, 1 May 2025 14:00:00 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [RFC PATCH 1/3] mm: introduce new .mmap_proto() f_op callback

On Thu, May 01, 2025 at 03:17:07PM +0300, Mike Rapoport wrote:
> On Thu, May 01, 2025 at 11:23:32AM +0100, Lorenzo Stoakes wrote:
> > On Wed, Apr 30, 2025 at 11:58:14PM +0200, David Hildenbrand wrote:
> > > On 30.04.25 21:54, Lorenzo Stoakes wrote:
> > > > Provide a means by which drivers can specify which fields of those
> > > > permitted to be changed should be altered to prior to mmap()'ing a
> > > > range (which may either result from a merge or from mapping an entirely new
> > > > VMA).
> > > >
> > > > Doing so is substantially safer than the existing .mmap() calback which
> > > > provides unrestricted access to the part-constructed VMA and permits
> > > > drivers and file systems to do 'creative' things which makes it hard to
> > > > reason about the state of the VMA after the function returns.
> > > >
> > > > The existing .mmap() callback's freedom has caused a great deal of issues,
> > > > especially in error handling, as unwinding the mmap() state has proven to
> > > > be non-trivial and caused significant issues in the past, for instance
> > > > those addressed in commit 5de195060b2e ("mm: resolve faulty mmap_region()
> > > > error path behaviour").
> > > >
> > > > It also necessitates a second attempt at merge once the .mmap() callback
> > > > has completed, which has caused issues in the past, is awkward, adds
> > > > overhead and is difficult to reason about.
> > > >
> > > > The .mmap_proto() callback eliminates this requirement, as we can update
> > > > fields prior to even attempting the first merge. It is safer, as we heavily
> > > > restrict what can actually be modified, and being invoked very early in the
> > > > mmap() process, error handling can be performed safely with very little
> > > > unwinding of state required.
> > > >
> > > > Update vma userland test stubs to account for changes.
> > > >
> > > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > >
> > >
> > > I really don't like the "proto" terminology. :)
> > >
> > > [yes, David and his naming :P ]
> > >
> > > No, the problem is that it is fairly unintuitive what is happening here.
> > >
> > > Coming from a different direction, the callback is trigger after
> > > __mmap_prepare() ... could we call it "->mmap_prepare" or something like
> > > that? (mmap_setup, whatever)
> > >
> > > Maybe mmap_setup and vma_setup_param? Just a thought ...
> >
> > Haha that's fine, I'm not sure I love 'proto' either to be honest, naming is
> > hard...
> >
> > I would rather not refer to VMA's at all to be honest, if I had my way, no
> > driver would ever have access to a VMA at all...
> >
> > But mmap_setup() or mmap_prepare() sound good!
>
> +1
>
> and struct vm_area_desc maybe?

That's nice actually thanks, will do!

>
> > >
> > >
> > > In general (although it's late in Germany), it does sound like an
> > > interesting approach.
> >
> > Thanks! Appreciate it :) I really want to attack this, as I _hate_ how we
> > effectively allow drivers to do _anything_ with VMAs like this.
> >
> > Yes, hate-driven development...
>
> Just move vm_area_struct to mm/internal.h and let them cope :-D

Haha oh man the dream. Though it'd be vma.h of course :P

>
> --
> Sincerely yours,
> Mike.

Cheers, Lorenzo