Re: [RFC 0/3] iommu/intel: Free empty page tables on unmaps

[Pasha Tatashin]

On Wed, Dec 20, 2023 at 11:16 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Dec 21, 2023 at 03:19:12AM +0000, Pasha Tatashin wrote:
> > This series frees empty page tables on unmaps. It intends to be a
> > low overhead feature.
> >
> > The read-writer lock is used to synchronize page table, but most of
> > time the lock is held is reader. It is held as a writer for short
> > period of time when unmapping a page that is bigger than the current
> > iova request. For all other cases this lock is read-only.
> >
> > page->refcount is used in order to track number of entries at each page
> > table.
>
> Have I not put enough DANGER signs up around the page refcount?
>
>  * If you want to use the refcount field, it must be used in such a way
>  * that other CPUs temporarily incrementing and then decrementing the
>  * refcount does not cause problems.  On receiving the page from
>  * alloc_pages(), the refcount will be positive.
>
> You can't use refcount for your purpose, and honestly I'm shocked you
> haven't seen any of your WARNings trigger.

Hi Matthew,

Thank you for looking at this.

Could you please explain exactly why refcount can't be used like this?

After alloc_page() refcount is set to 1, we never reduce it to 0,
every new entry in a page table adds 1, so we get up-to 513, that is
why I added warn like this: WARN_ON_ONCE(rc > 513 || rc < 2); to
dma_set_pte() macro. When refcount == 1, we know that the page table
is empty, and can be added to a freelist for a delayed freeing.

What is wrong with using refcount for a scalable way of keeping the
track of number of entries in a iommu page table? Is there a better
way that I should use?

Thank you,
Pasha