linux-kernel - Re: [PATCH 12/16] mm: update resctl to use mmap_prepare, mmap_complete, mmap

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Message-ID: <fb9ce6c6-913e-491a-99c6-0bb38f0d1d37@lucifer.local>
Date: Mon, 8 Sep 2025 14:40:21 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, Matthew Wilcox <willy@...radead.org>,
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Subject: Re: [PATCH 12/16] mm: update resctl to use mmap_prepare,
 mmap_complete, mmap_abort

On Mon, Sep 08, 2025 at 10:24:47AM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 12:10:43PM +0100, Lorenzo Stoakes wrote:
> > resctl uses remap_pfn_range(), but holds a mutex over the
> > operation. Therefore, establish the mutex in mmap_prepare(), release it in
> > mmap_complete() and release it in mmap_abort() should the operation fail.
>
> The mutex can't do anything relative to remap_pfn, no reason to hold it.
>
> > @@ -1053,15 +1087,11 @@ static int pseudo_lock_dev_mmap(struct file *filp, struct vm_area_struct *vma)
> >  		return -ENOSPC;
> >  	}
> >
> > -	memset(plr->kmem + off, 0, vsize);
> > +	/* No CoW allowed so don't need to specify pfn. */
> > +	remap_pfn_range_prepare(desc, 0);
>
> This would be a good place to make a more generic helper..
>
>  ret = remap_pfn_no_cow(desc, phys);

Ha, funny I suggested a _no_cow() thing earlier :) seems we are agreed on that
then!

Presumably you mean remap_pfn_no_cow_prepare()?

>
> And it can consistently check for !shared internally.
>
> Store phys in the desc and use common code to trigger the PTE population
> during complete.

We can use mmap_context for this, I guess it's not a terrible idea to set .pfn
but I just don't want to add any confusion as to what doing that means in
the non-generic mmap_complete case.

>
> Jason

Cheers, Lorenzo