linux-kernel - Re: [PATCH 3/3] mm/vma: remove mmap() retry merge

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Message-ID: <533100e9-8eed-4f5f-a02b-47a7acb2a646@lucifer.local>
Date: Fri, 9 May 2025 11:38:56 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "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>,
        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>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 3/3] mm/vma: remove mmap() retry merge

On Fri, May 09, 2025 at 11:45:45AM +0200, David Hildenbrand wrote:
> On 07.05.25 13:03, Lorenzo Stoakes wrote:
> > We have now introduced a mechanism that obviates the need for a reattempted
> > merge via the mmap_prepare() file hook, so eliminate this functionality
> > altogether.
> >
> > The retry merge logic has been the cause of a great deal of complexity in
> > the past and required a great deal of careful manoeuvring of code to ensure
> > its continued and correct functionality.
> >
> > It has also recently been involved in an issue surrounding maple tree
> > state, which again points to its problematic nature.
> >
> > We make it much easier to reason about mmap() logic by eliminating this and
> > simply writing a VMA once. This also opens the doors to future optimisation
> > and improvement in the mmap() logic.
> >
> > For any device or file system which encounters unwanted VMA fragmentation
> > as a result of this change (that is, having not implemented .mmap_prepare
> > hooks), the issue is easily resolvable by doing so.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: David Hildenbrand <david@...hat.com>

Thanks!

>
> --
> Cheers,
>
> David / dhildenb
>