Re: [PATCH 12/16] mm: update resctl to use mmap_prepare, mmap_complete, mmap_abort

Lorenzo Stoakes
2025-09-08T14:03:05Z

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