[PATCH 10/17] prmem: documentation

[Andy Lutomirski]

On Tue, Oct 30, 2018 at 2:36 PM Matthew Wilcox <willy at infradead.org> wrote:
>
> On Tue, Oct 30, 2018 at 10:43:14PM +0200, Igor Stoppa wrote:
> > On 30/10/2018 21:20, Matthew Wilcox wrote:
> > > > > So the API might look something like this:
> > > > >
> > > > >         void *p = rare_alloc(...);      /* writable pointer */
> > > > >         p->a = x;
> > > > >         q = rare_protect(p);            /* read-only pointer */
> >
> > With pools and memory allocated from vmap_areas, I was able to say
> >
> > protect(pool)
> >
> > and that would do a swipe on all the pages currently in use.
> > In the SELinux policyDB, for example, one doesn't really want to
> > individually protect each allocation.
> >
> > The loading phase happens usually at boot, when the system can be assumed to
> > be sane (one might even preload a bare-bone set of rules from initramfs and
> > then replace it later on, with the full blown set).
> >
> > There is no need to process each of these tens of thousands allocations and
> > initialization as write-rare.
> >
> > Would it be possible to do the same here?
>
> What Andy is proposing effectively puts all rare allocations into
> one pool.  Although I suppose it could be generalised to multiple pools
> ... one mm_struct per pool.  Andy, what do you think to doing that?

Hmm.  Let's see.

To clarify some of this thread, I think that the fact that rare_write
uses an mm_struct and alias mappings under the hood should be
completely invisible to users of the API.  No one should ever be
handed a writable pointer to rare_write memory (except perhaps during
bootup or when initializing a large complex data structure that will
be rare_write but isn't yet, e.g. the policy db).

For example, there could easily be architectures where having a
writable alias is problematic.  On such architectures, an entirely
different mechanism might work better.  And, if a tool like KNOX ever
becomes a *part* of the Linux kernel (hint hint!)

If you have multiple pools and one mm_struct per pool, you'll need a
way to find the mm_struct from a given allocation.  Regardless of how
the mm_structs are set up, changing rare_write memory to normal memory
or vice versa will require a global TLB flush (all ASIDs and global
pages) on all CPUs, so having extra mm_structs doesn't seem to buy
much.

(It's just possible that changing rare_write back to normal might be
able to avoid the flush if the spurious faults can be handled
reliably.)