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

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Message-ID: <20180824113214.GK24142@hirez.programming.kicks-ass.net>
Date:   Fri, 24 Aug 2018 13:32:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Benjamin Herrenschmidt <benh@....ibm.com>,
        Nick Piggin <npiggin@...il.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, 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")

yuck yuck yuck. That needs fixing.