Re: [PATCH 2/2] s390/tlb: convert to generic mmu_gather

[Peter Zijlstra]

On Wed, Sep 19, 2018 at 04:28:09PM +0200, Martin Schwidefsky wrote:
> On Wed, 19 Sep 2018 14:38:49 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > On Tue, Sep 18, 2018 at 02:51:51PM +0200, Martin Schwidefsky wrote:

> > > +        page_table_free_rcu(tlb, (unsigned long *) pte, address);  
> > 
> > (whitespace damage, fixed)
> > 
> > Also, could you perhaps explain the need for that
> > page_table_alloc/page_table_free code? That is, I get the comment about
> > using 2K page-table fragments out of 4k physical page, but why this
> > custom allocator instead of kmem_cache? It feels like there's a little
> > extra complication, but it's not immediately obvious what.
> 
> The kmem_cache code uses the fields of struct page for its tracking.
> pgtable_page_ctor uses the same fields, e.g. for the ptl. Last time
> I tried to convert the page_table_alloc/page_table_free to kmem_cache
> it just crashed. Plus the split of 4K pages into 2 2K fragments is
> done on a per mm basis, that should help a little bit with fragmentation.

Fair enough, thanks for the information.

> > It's that ASCE limit that makes it impossible to use the generic
> > helpers, right?
> 
> There are two problems, one of them is related to the ASCE limit:
> 
> 1) s390 supports 4 different page table layouts. 2-levels (2^31 bytes) for 31-bit compat,
>    3-levels (2^42 bytes) as the default for 64-bit, 4-levels (2^53) if 4 tera-bytes are
>    not enough and 5-levels (2^64) for the bragging rights.
>    The pxd_free_tlb() turn into nops if the number of page table levels require it.

Shiny, I think we (x86) have to choose at boot time which paging mode we
want and have to stick to it.

> 2) The mm->context.flush_mm indication.
>    That goes back to this beauty in the architecture:
> 
>     * "A valid table entry must not be changed while it is attached
>     * to any CPU and may be used for translation by that CPU except to
>     * (1) invalidate the entry by using INVALIDATE PAGE TABLE ENTRY,
>     * or INVALIDATE DAT TABLE ENTRY, (2) alter bits 56-63 of a page
>     * table entry, or (3) make a change by means of a COMPARE AND SWAP
>     * AND PURGE instruction that purges the TLB."
> 
>    If one CPU is doing a mmu_gather page table operation on the only active thread
>    in the system the individual page table updates are done in a lazy fashion with
>    simple stores. If a second CPU picks up another thread for execution, the
>    attach_count is increased and the page table updates are done with IPTE/IDTE
>    from now on. But there might by TLBs of around that are not flushed yet.
>    We may *not* let the second CPU see these TLBs, otherwise the CPU may start an
>    instruction, then loose the TLB without being able to recreate it. Due to that
>    the CPU can end up with a half finished instruction it can not roll back nor
>    complete, ending in a check-stop. The simplest example is MVC with a length
>    of e.g. 256 bytes. The instruction has to complete with all 256 bytes moved,
>    or no bytes may have at all.
>    That is where the mm->context.flush_mm indication comes into play, if the
>    second CPU finds the bit set at the time it attaches a thread, it will to
>    an IDTE for flush all TLBs for the mm.

Oh man.. what fun. Still, this bit could easily be set in the
__*_free_tlb() functions afaict. Still 1) above is enough.

Thanks!