Re: [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment

[Peter Zijlstra]

On Thu, Sep 13, 2018 at 12:30:14PM +0200, Martin Schwidefsky wrote:

> > + * The mmu_gather data structure is used by the mm code to implement the
> > + * correct and efficient ordering of freeing pages and TLB invalidations.
> > + *
> > + * This correct ordering is:
> > + *
> > + *  1) unhook page
> > + *  2) TLB invalidate page
> > + *  3) free page
> > + *
> > + * That is, we must never free a page before we have ensured there are no live
> > + * translations left to it. Otherwise it might be possible to observe (or
> > + * worse, change) the page content after it has been reused.
> > + *
> 
> This first comment already includes the reason why s390 is probably better off
> with its own mmu-gather implementation. It depends on the situation if we have
> 
> 1) unhook the page and do a TLB flush at the same time
> 2) free page
> 
> or
> 
> 1) unhook page
> 2) free page
> 3) final TLB flush of the whole mm

that's the fullmm case, right?

> A variant of the second order we had in the past is to do the mm TLB flush first,
> then the unhooks and frees of the individual pages. The are some tricky corners
> switching between the two variants, see finish_arch_post_lock_switch.
> 
> The point is: we *never* have the order 1) unhook, 2) TLB invalidate, 3) free.
> If there is concurrency due to a multi-threaded application we have to do the
> unhook of the page-table entry and the TLB flush with a single instruction.

You can still get the thing you want if for !fullmm you have a no-op
tlb_flush() implementation, assuming your arch page-table frobbing thing
has the required TLB flush in.

Note that that's not utterly unlike how the PowerPC/Sparc hash things
work, they clear and invalidate entries different from others and don't
use the mmu_gather tlb-flush.