linux-kernel - Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE

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Date:   Mon, 27 Aug 2018 15:00:08 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will.deacon@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Benjamin Herrenschmidt <benh@....ibm.com>,
        Andrew Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        Jann Horn <jannh@...gle.com>,
        Adin Scannell <ascannell@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        David Miller <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for
 RCU_TABLE_FREE

On Fri, 24 Aug 2018 13:39:53 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, Aug 24, 2018 at 01:32:14PM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 24, 2018 at 10:47:17AM +0200, Peter Zijlstra wrote:  
> > > On Thu, Aug 23, 2018 at 02:39:59PM +0100, Will Deacon wrote:  
> > > > The only problem with this approach is that we've lost track of the granule
> > > > size by the point we get to the tlb_flush(), so we can't adjust the stride of
> > > > the TLB invalidations for huge mappings, which actually works nicely in the
> > > > synchronous case (e.g. we perform a single invalidation for a 2MB mapping,
> > > > rather than iterating over it at a 4k granule).
> > > > 
> > > > One thing we could do is switch to synchronous mode if we detect a change in
> > > > granule (i.e. treat it like a batch failure).  
> > > 
> > > We could use tlb_start_vma() to track that, I think. Shouldn't be too
> > > hard.  
> > 
> > Hurm.. look at commit:
> > 
> >   e77b0852b551 ("mm/mmu_gather: track page size with mmu gather and force flush if page size change")  
> 
> Ah, good, it seems that already got cleaned up a lot. But it all moved
> into the power code.. blergh.

I lost track of what the problem is here?

For powerpc, tlb_start_vma is not the right API to use for this because
it wants to deal with different page sizes within a vma.

Thanks,
Nick