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

[Pasha Tatashin]

On Thu, Dec 21, 2023 at 9:06 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Dec 21, 2023 at 12:42:41AM -0500, Pasha Tatashin wrote:
> > On Thu, Dec 21, 2023 at 12:13 AM Pasha Tatashin
> > <pasha.tatashin@xxxxxxxxxx> wrote:
> > >
> > > 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
> >
> > I guess, what you mean is that other CPUs could temporarily
> > increase/decrease refcount outside of IOMMU management, do you have an
> > example of why that would happen? I could remove the above warning,
> > and in the worst case we would miss an opportunity to free a page
> > table during unmap, not a big deal, it can be freed during another
> > map/unmap event. Still better than today, where we never free them
> > during unmaps.
>
> Both GUP-fast and the page cache will find a page under RCU protection,
> inc it's refcount if not zero, check the page is still the one they were
> looking for, and if not will dec the refcount again.  That means if a
> page has been in the page cache or process page tables and you can't
> guarantee that all CPUs have been through the requisite grace periods,
> you might see the refcount increased.

Interesting scenario, it sounds like this could only happen for a
short period of time at the beginning of the life of a page in the
IOMMU Page Table.


> I'm not prepared to make a guarantee that these are the only circumstances
> under which you'll see a temporarily higher refcount than you expect.
> Either currently or in the future.  If you use the refcount as anything
> other than a refcount, you're living dangerously.  And if you think that
> you'll be the one to do the last refcount put, you're not necessarily
> correct (see the saga around __free_pages() which ended up as commit
> e320d3012d25 fixed by 462a8e08e0e6 (which indicates the rare race does
> actually happen)).
>
> Now, it seems like from your further explanation that the consequence
> of getting this wrong is simply that you fail to free the page early.
> That seems OK, but I insist that you insert some comments explaining
> what is going on and why it's safe so somebody auditing uses of refcount
> doesn't have to reanalyse the whole thing for themself.  Or worse that
> somebody working on the iommu sees this and thinks they can "improve"
> on it.

Yes, I can add detailed comments explaining how refcount is used here.

Alternatively, I was thinking of using mapcount:

>From mm_types.h:
  * If your page will not be mapped to userspace, you can also use the
four
  * bytes in the mapcount union, but you must call
page_mapcount_reset()
  * before freeing it.

It sounds like we can safely use _mapcount for our needs, and do
page_mapcount_reset() before freeing pages.

Pasha