linux-kernel - Re: [RFC PATCH 0/3] eliminate mmap() retry merge, add .mmap

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Message-ID: <c2254436-008c-4adc-a144-78dc81d8607e@lucifer.local>
Date: Thu, 1 May 2025 11:27:51 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jann Horn <jannh@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.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>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC PATCH 0/3] eliminate mmap() retry merge, add .mmap_proto
 hook

On Wed, Apr 30, 2025 at 11:29:46PM +0200, Jann Horn wrote:
> On Wed, Apr 30, 2025 at 9:59 PM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> > A driver can specify either .mmap_proto(), .mmap() or both. This provides
> > maximum flexibility.
>
> Just to check I understand the intent correctly: The idea here is that
> .mmap_proto() is, at least for now, only for drivers who want to
> trigger merging, right? If a driver doesn't need merging, the normal
> .mmap() handler can still do all the things it was able to do before
> (like changing the ->vm_file pointer through vma_set_file(), or
> changing VMA flags in allowed ways)?

No, the intent is that this form the basis of an entirely new set of callbacks
to use _instead_ of .mmap().

The vma_set_file() semantics would need to be changed actually, I will update
logic to do this implicitly when the user sets vma_proto (or whatever this gets
renamed to :P)->file - we can do this automagically.

However the first use is indeed to be able to remove this merge retry. I have
gone through .mmap() callbacks and identified the only one that seems
meaningfully to care, so it's a great first use.

Two birds with one stone, and forming the foundatio for future work to prevent
drivers from having carte blache to do whatever on .mmap() wrt vma's.