linux-kernel - Re: [PATCH 06/16] mm: introduce the f_op->mmap_complete, mmap

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Message-ID: <0df59b0c-dd8d-4087-9ddf-3659326f57e9@lucifer.local>
Date: Mon, 8 Sep 2025 14:19:14 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH 06/16] mm: introduce the f_op->mmap_complete, mmap_abort
 hooks

On Mon, Sep 08, 2025 at 09:55:26AM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 12:10:37PM +0100, Lorenzo Stoakes wrote:
> > We have introduced the f_op->mmap_prepare hook to allow for setting up a
> > VMA far earlier in the process of mapping memory, reducing problematic
> > error handling paths, but this does not provide what all
> > drivers/filesystems need.
> >
> > In order to supply this, and to be able to move forward with removing
> > f_op->mmap altogether, introduce f_op->mmap_complete.
> >
> > This hook is called once the VMA is fully mapped and everything is done,
> > however with the mmap write lock and VMA write locks held.
> >
> > The hook is then provided with a fully initialised VMA which it can do what
> > it needs with, though the mmap and VMA write locks must remain held
> > throughout.
> >
> > It is not intended that the VMA be modified at this point, attempts to do
> > so will end in tears.
>
> The commit message should call out if this has fixed the race
> condition with unmap mapping range and prepopulation in mmap()..

To be claer, this isn't the intent of the series, the intent is to make it
possible for mmap_prepare to replace mmap. This is just a bonus :)

Looking at the discussion in [0] it seems the issue was that .mmap() is
called before the vma is actually correctly inserted into the maple tree.

This is no longer the case, we call .mmap_complete() once the VMA is fully
established, but before releasing the VMA/mmap write lock.

This should, presumably, resolve the race as stated?

I can add some blurb about this yes.


[0]:https://lore.kernel.org/linux-mm/20250801162930.GB184255@nvidia.com/


>
> > @@ -793,6 +793,11 @@ struct vm_area_desc {
> >  	/* Write-only fields. */
> >  	const struct vm_operations_struct *vm_ops;
> >  	void *private_data;
> > +	/*
> > +	 * A user-defined field, value will be passed to mmap_complete,
> > +	 * mmap_abort.
> > +	 */
> > +	void *mmap_context;
>
> Seem strange, private_data and mmap_context? Something actually needs
> both?

We are now doing something _new_ - we're splitting an operation that was
never split before.

Before a hook implementor could rely on there being state throughout the
_entire_ operation. But now they can't.

And they may already be putting context into private_data, which then gets
put into vma->vm_private_data for a VMA added to the maple tree and made
accessible.

So it is appropriate and convenient to allow for the transfer of state
between the two, and I already implement logic that does this.

>
> Jason

Cheers, Lorenzo