linux-kernel - Re: [PATCH 00/10] convert the majority of file systems to mmap

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Message-ID: <3cf76128-390a-4ef2-85a7-e3ee21ba04b5@lucifer.local>
Date: Fri, 1 Aug 2025 15:12:48 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Jens Axboe <axboe@...nel.dk>, Christian Brauner <brauner@...nel.org>,
        Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        Kees Cook <kees@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 00/10] convert the majority of file systems to
 mmap_prepare

[culling cc list down because for it was insane, +David]

(apologies if I cut anybody who wants to be involved in discussion, just
being practical here).

On Fri, Aug 01, 2025 at 11:00:57AM -0300, Jason Gunthorpe wrote:
> On Mon, Jun 16, 2025 at 08:33:19PM +0100, Lorenzo Stoakes wrote:
>
> > The intent is to gradually deprecate f_op->mmap, and in that vein this
> > series coverts the majority of file systems to using f_op->mmap_prepare.
>
> I saw this on lwn and just wanted to give a little bit of thought on
> this topic..

Thanks, sure.

>
> It looks to me like we need some more infrastructure to convert
> anything that uses remap_pfn/etc in the mmap() callback

Yes absolutely :) I realised we needed extra infra, and later noticed (and
in discussion with others) that remap in particular would be a pain.

I wanted to lay the groundwork first, as we could safely do this, convert
nearly all file systems to .mmap_prepare(), and get a big win on that, then
take it further in the next cycle, which I was planning to do.

>
> I would like to suggest we add a vma->prepopulate() callback which is
> where the remap_pfn should go. Once the VMA is finalized and fully
> operational the vma_ops have the opportunity to prepopulate any PTEs.

I assume you mean vma->vm_ops->prepopulate ?

We also have to think about other places where we prepopulate also, for
instance the perf mmap call now prepopulates (ahem that was me).

So I do like this as a generalisation.

>
> This could then actually be locked properly so it is safe with
> concurrent unmap_mapping_range() (current mmap callback is not safe)

Which lock in particular is problematic? You'd want to hold an rmap write
lock to avoid racing zap?

>
> Jason

BTW I'll loop you in on discussion/series here. I think what you outline is
likely how this will work.

Cheers, Lorenzo