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

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Message-ID: <7ab1743b-8826-44e8-ac11-283731ef51e1@redhat.com>
Date: Wed, 30 Apr 2025 23:58:14 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 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 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 ...

In general (although it's late in Germany), it does sound like an 
interesting approach.

How feasiable is it to remove ->mmap in the long run, and would we maybe 
need other callbacks to make that possible?

-- 
Cheers,

David / dhildenb