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

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Date: Thu, 21 Dec 2023 00:42:41 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rientjes@...gle.com, dwmw2@...radead.org, 
	baolu.lu@...ux.intel.com, joro@...tes.org, will@...nel.org, 
	robin.murphy@....com, iommu@...ts.linux.dev
Subject: Re: [RFC 0/3] iommu/intel: Free empty page tables on unmaps

On Thu, Dec 21, 2023 at 12:13 AM Pasha Tatashin
<pasha.tatashin@...een.com> wrote:
>
> On Wed, Dec 20, 2023 at 11:16 PM Matthew Wilcox <willy@...radead.org> 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.

Pasha