Re: [PATCH v2 05/10] x86/mm: Rework lazy TLB mode and TLB freshness tracking

[Juergen Gross]

On 14/06/17 06:56, Andy Lutomirski wrote:
> x86's lazy TLB mode used to be fairly weak -- it would switch to
> init_mm the first time it tried to flush a lazy TLB.  This meant an
> unnecessary CR3 write and, if the flush was remote, an unnecessary
> IPI.
> 
> Rewrite it entirely.  When we enter lazy mode, we simply remove the
> cpu from mm_cpumask.  This means that we need a way to figure out
> whether we've missed a flush when we switch back out of lazy mode.
> I use the tlb_gen machinery to track whether a context is up to
> date.
> 
> Note to reviewers: this patch, my itself, looks a bit odd.  I'm
> using an array of length 1 containing (ctx_id, tlb_gen) rather than
> just storing tlb_gen, and making it at array isn't necessary yet.
> I'm doing this because the next few patches add PCID support, and,
> with PCID, we need ctx_id, and the array will end up with a length
> greater than 1.  Making it an array now means that there will be
> less churn and therefore less stress on your eyeballs.
> 
> NB: This is dubious but, AFAICT, still correct on Xen and UV.
> xen_exit_mmap() uses mm_cpumask() for nefarious purposes and this
> patch changes the way that mm_cpumask() works.  This should be okay,
> since Xen *also* iterates all online CPUs to find all the CPUs it
> needs to twiddle.

There is a allocation failure path in xen_drop_mm_ref() which might
be wrong with this patch. As this path should be taken only very
unlikely I'd suggest to remove the test for mm_cpumask() bit zero in
this path.


Juergen